Revert 285259 "Avoid processing mutations when ChromeVox active ..."

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

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=285299

[Alexei Svitkine (slow), 2014-07-24 16:52:03]

Committed patchset #1 manually as r285299.