Revert 285299 "Revert 285259 "Avoid processing mutations when Ch..."

Revert 285299 "Revert 285259 "Avoid processing mutations when Ch..."

Looks like the revert didn't help.

> Revert 285259 "Avoid processing mutations when ChromeVox active ..."
> 
> Suspected to have caused CvoxBrailleUtilUnitTest.TextFieldSelection to
> start being flaky. Sample failure output:
> 
> CvoxBrailleUtilUnitTest.TextFieldSelection (run #1):
> [ RUN      ] CvoxBrailleUtilUnitTest.TextFieldSelection
> [5693:5693:0724/085516:WARNING:chrome_browser_main_chromeos.cc(275)] Running as stub user with profile dir: test-user
> Xlib:  extension "RANDR" missing on display ":9".
> [5693:5822:0724/085517:WARNING:local_extension_cache.cc(199)] Extensions will not be installed from update URLs until /var/cache/external_cache/.initialized exists.
> [5693:5771:0724/085518:WARNING:raw_channel_posix.cc(214)] recvmsg: Connection reset by peer
> [5693:5693:0724/085518:INFO:CONSOLE(1283)] "Running TestCase CvoxBrailleUtilUnitTest.TextFieldSelection", source: test_api.js (1283)
> [5693:5693:0724/085518:ERROR:web_ui_test_handler.cc(74)] undefinedFailed: RUN_TEST_F("CvoxBrailleUtilUnitTest","TextFieldSelection")
> Expected 'NavBraille(text=": strawberry edtxt"  startIndex="4"  endIndex="7")' to be eqJSON({"text":{"string_":": strawberry edtxt","spans_":[]},"startIndex":0,"endIndex":1})
> obj/chrome/browser/resources/chromeos/chromevox/chromevox_tests.gen/common/braille_util_test-gen.cc:407: Failure
> Value of: RunJavascriptTestF(false, "CvoxBrailleUtilUnitTest", "TextFieldSelection")
>   Actual: false
> Expected: true
> [5693:5771:0724/085518:WARNING:raw_channel_posix.cc(214)] recvmsg: Connection reset by peer
> [  FAILED  ] CvoxBrailleUtilUnitTest.TextFieldSelection, where TypeParam =  and GetParam() =  (1472 ms)
> 
> 
> > Avoid processing mutations when ChromeVox active indicator moves.
> > 
> > This makes touch exploration a bit more responsive; a lot of the processing
> > time was spent responding to the mutations generated from the active
> > indicator.
> > 
> > BUG=377040
> > 
> > Review URL: https://codereview.chromium.org/405433010
> 
> TBR=dmazzoni@chromium.org
> 
> Review URL: https://codereview.chromium.org/418893002

TBR=asvitkine@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=285330

[Alexei Svitkine (slow), 2014-07-24 18:14:01]

Committed patchset #1 manually as r285330.