Revert of IDL: Support optional argument default value syntax (https://codereview.chromium.org/312683005/)

Reason for revert:
This patch is the most likely candidate for browser test failures: http://build.chromium.org/p/chromium.webkit/builders/Linux%20Tests%20%28dbg%29/builds/2905

BrowserTestBase signal handler received SIGTERM. Backtrace:
#0 0x7f2c2a201a1d base::debug::StackTrace::StackTrace()
#1 0x00000460caf2 content::(anonymous namespace)::DumpStackTraceSignalHandler()
#2 0x7f2c232cd4a0 \u003Cunknown>
#3 0x7f2c2337f313 __poll
#4 0x7f2c240b6036 \u003Cunknown>
#5 0x7f2c240b6164 g_main_context_iteration
#6 0x7f2c2a1c9b45 base::MessagePumpGlib::Run()
#7 0x7f2c2a29eed7 base::MessageLoop::RunHandler()
#8 0x7f2c2a2f69d8 base::RunLoop::Run()
#9 0x00000466bd59 content::RunThisRunLoop()
#10 0x00000466c1b8 content::MessageLoopRunner::Run()
#11 0x000004610946 content::TitleWatcher::WaitAndGetTitle()
#12 0x0000011fdf00 MediaBrowserTest::RunTest()
#13 0x0000011fdc00 MediaBrowserTest::RunMediaTestPage()
#14 0x0000011ec98b EncryptedMediaTestBase::RunEncryptedMediaTestPage()
#15 0x0000011ee5fa EncryptedMediaTest::TestConfigChange()
#16 0x0000011eb73a EncryptedMediaTest_ConfigChangeVideo_Test::RunTestOnMainThread()
...

Please look into it. An alterate possible candidate is  https://codereview.chromium.org/327553002 but I think that's less likely.

Original issue's description:
> IDL: Support optional argument default value syntax
> 
> Adds support for parsing default values of different types, but
> only handles null default values when generating code.
> 
> Replaces existing
> 
>   [Default=Null] optional SomeInterface arg
>   [Default=NullString] optional DOMString arg
> 
> with the now equivalent
> 
>   optional SomeInterface arg = null
>   optional DOMString arg = null
> 
> in IDL files, and drops support for those [Default] attributes.
> 
> No changes to generated code.
> 
> BUG=258153
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=176200

TBR=nbarth@chromium.org,haraken@chromium.org,pavan.e@samsung.com,jsbell@chromium.org,jl@opera.com
NOTREECHECKS=true
NOTRY=true
BUG=258153

Review URL: https://codereview.chromium.org/339683002

git-svn-id: svn://svn.chromium.org/blink/trunk@176220 bbb929c8-8fbe-4397-9dbb-9b2b20218538