Revert of Revert of Make start/stop loading notifications per-frame (https://codereview.chromium.org/215533004/)

Reason for revert:
r170192 was not a culprit.


Original issue's description:
> Revert of Make start/stop loading notifications per-frame (https://codereview.chromium.org/183793002/)
> 
> Reason for revert:
> Possibly causing content_browsertests EndToEnd to fail.
> 
> See: http://build.chromium.org/p/chromium.webkit/builders/Linux%20Tests%20%28dbg%29/builds/2095/steps/content_browsertests/logs/stdio
> 
> Output:
> [20917:20917:0327/122353:3268961829:INFO:CONSOLE(0)] "Uncaught TypeError: Cannot convert undefined or null to object", source: http://127.0.0.1:34379/files/web_ui_mojo.html (0)
> [20917:20917:0327/122353:3269022437:INFO:CONSOLE(28)] "Uncaught TypeError: undefined is not a function", source: http://127.0.0.1:34379/files/web_ui_mojo.js (28)
> BrowserTestBase signal handler received SIGTERM. Backtrace:
> #0 0x000000a593be base::debug::StackTrace::StackTrace()
> #1 0x000002bfc17b content::(anonymous namespace)::DumpStackTraceSignalHandler()
> #2 0x7f9c3e4ab4a0 <unknown>
> #3 0x7f9c3e55da43 __poll
> #4 0x7f9c42aabff6 <unknown>
> #5 0x7f9c42aac124 g_main_context_iteration
> #6 0x000000aaaf5f base::MessagePumpGlib::RunWithDispatcher()
> #7 0x000000a8e802 base::RunLoop::Run()
> #8 0x00000058256e content::(anonymous namespace)::WebUIMojoTest_EndToEnd_Test::RunTestOnMainThread()
> 
> Original issue's description:
> > Make start/stop loading notifications per-frame
> > 
> > Also, make ProgressTracker a frame-level concept rather than a page-level concept and merge it with FrameLoader's FrameProgressTracker helper. Send per-frame progress change notifications accordingly.
> > 
> > This depends on https://codereview.chromium.org/180113003/ landing in chromium.
> > 
> > BUG=347643
> > 
> > Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=170192
> 
> TBR=abarth@chromium.org,japhet@chromium.org
> NOTREECHECKS=true
> NOTRY=true
> BUG=347643
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=170218

TBR=abarth@chromium.org,japhet@chromium.org,enne@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=347643

Review URL: https://codereview.chromium.org/216083002

git-svn-id: svn://svn.chromium.org/blink/trunk@170248 bbb929c8-8fbe-4397-9dbb-9b2b20218538