Revert "Allow XHR timeout attribute to be overridden after send(), per spec"

This reverts https://codereview.chromium.org/273993002, which might have caused
the following ASAN failures:

http://build.chromium.org/p/chromium.memory/builders/Linux%20ASan%20LSan%20Tests%20%281%29/builds/3314/steps/browser_tests/logs/MimeHandler

==22764==ERROR: AddressSanitizer: heap-use-after-free on address 0x612000001c78 at pc 0x870ca08 bp 0x7fff7edd07f0 sp 0x7fff7edd07e8
WRITE of size 8 at 0x612000001c78 thread T0 (browser_tests)
[22723:22723:0621/141550:INFO:nacl_browsertest_util.cc(132)] |||| [STARTUP]
    0 0x870ca07 in handleSuccessfulFinish third_party/WebKit/Source/core/loader/DocumentThreadableLoader.cpp:393
    1 0x870ca07 in notifyFinished third_party/WebKit/Source/core/loader/DocumentThreadableLoader.cpp:382
    2 0x870ca07 in non-virtual thunk to WebCore::DocumentThreadableLoader::notifyFinished(WebCore::Resource*) third_party/WebKit/Source/core/loader/DocumentThreadableLoader.cpp:383
    3 0x84b1129 in WebCore::Resource::checkNotify() third_party/WebKit/Source/core/fetch/Resource.cpp:198
    4 0x84b23cc in WebCore::Resource::finish(double) third_party/WebKit/Source/core/fetch/Resource.cpp:258
    5 0x84e17b5 in WebCore::ResourceLoader::didFinishLoading(blink::WebURLLoader*, double, long) third_party/WebKit/Source/core/fetch/ResourceLoader.cpp:425
    6 0xca68742 in content::WebURLLoaderImpl::Context::OnCompletedRequest(int, bool, bool, std::string const&, base::TimeTicks const&, long) content/child/web_url_loader_impl.cc:637
    7 0xca3400a in content::ResourceDispatcher::OnRequestComplete(int, ResourceMsg_RequestCompleteData const&) content/child/resource_dispatcher.cc:561
    8 0xca2fca3 in DispatchToMethod\u003Ccontent::ResourceDispatcher, void (content::ResourceDispatcher::*)(int, const ResourceMsg_RequestCompleteData &), int, ResourceMsg_RequestCompleteData> base/tuple.h:555
    9 0xca2fca3 in Dispatch\u003Ccontent::ResourceDispatcher, content::ResourceDispatcher, void, void (content::ResourceDispatcher::*)(int, const ResourceMsg_RequestCompleteData &)> content/common/resource_messages.h:299
    10 0xca2fca3 in content::ResourceDispatcher::DispatchMessage(IPC::Message const&) content/child/resource_dispatcher.cc:709
    11 0xca2dc61 in content::ResourceDispatcher::OnMessageReceived(IPC::Message const&) content/child/resource_dispatcher.cc:319
    12 0xc99d476 in content::ChildThread::OnMessageReceived(IPC::Message const&) content/child/child_thread.cc:441
<truncated, full output is in gzipped JSON output at end of step>
17 0x000002e280e7 ChromeBrowserMainParts::PreMainMessageLoopRun()
18 0x0000058a5538 content::BrowserMainLoop::PreMainMessageLoopRun()
19 0x000005bf69f8 content::StartupTaskRunner::RunAllTasksNow()
20 0x0000058a1c01 content::BrowserMainLoop::CreateStartupTasks()
21 0x000005d8c134 content::BrowserMainRunnerImpl::Initialize()
22 0x00000eee3cc1 content::BrowserMain()
23 0x00000ee1f601 content::ContentMainRunnerImpl::Run()
24 0x00000ee1c7b0 content::ContentMain()
25 0x00000cde7400 content::BrowserTestBase::SetUp()
26 0x000003402934 InProcessBrowserTest::SetUp()
27 0x000003e70a82 testing::Test::Run()
28 0x000003e72e1a testing::TestInfo::Run()
29 0x000003e73ba7 testing::TestCase::Run()
30 0x000003e86cbb testing::internal::UnitTestImpl::RunAllTests()
31 0x000003e862f1 testing::UnitTest::Run()
32 0x0000035a55dd base::TestSuite::Run()
33 0x000002116a75 ChromeBrowserTestSuiteRunner::RunTestSuite()
34 0x00000ce61a0a content::LaunchTests()
35 0x0000033ffc0c LaunchChromeTests()
36 0x000002116969 main

BUG=

Review URL: https://codereview.chromium.org/333823006

git-svn-id: svn://svn.chromium.org/blink/trunk@176702 bbb929c8-8fbe-4397-9dbb-9b2b20218538