Revert of Disable speech synthesis API for content/ layer.

Revert of Disable speech synthesis API for content/ layer. (https://codereview.chromium.org/355183002/)

Reason for revert:
This breaks layout tests that are enumerating window properties:

 PASS window.cached_scrollbars.visible is false
-PASS window.cached_speechSynthesis.onvoiceschanged is null
-PASS window.cached_speechSynthesis.paused is false
-PASS window.cached_speechSynthesis.pending is false
-PASS window.cached_speechSynthesis.speaking is false
 PASS window.cached_statusbar.visible is false

http://test-results.appspot.com/dashboards/flakiness_dashboard.html#group=%40ToT%20Blink&tests=fast/dom/Window/property-access-on-cached-properties-after-frame-navigated.html,fast/dom/Window/property-access-on-cached-properties-after-frame-removed-and-gced.html,fast/dom/Window/property-access-on-cached-properties-after-frame-removed.html,fast/dom/Window/property-access-on-cached-window-after-frame-navigated.html,fast/dom/Window/property-access-on-cached-window-after-frame-removed-and-gced.html,fast/dom/Window/property-access-on-cached-window-after-frame-removed.html,fast/speechsynthesis/speech-synthesis-boundary-events.html,fast/speechsynthesis/speech-synthesis-cancel-twice.html,fast/speechsynthesis/speech-synthesis-cancel.html,fast/speechsynthesis/speech-synthesis-gc-utterance-crash.html,fast/speechsynthesis/speech-synthesis-pause-resume.html,fast/speechsynthesis/speech-synthesis-speak-invalid-argument-throws.html,fast/speechsynthesis/speech-synthesis-speak.html,fast/speechsynthesis/speech-synthesis-utterance-uses-voice.html,fast/speechsynthesis/speech-synthesis-voices-changed.html,fast/speechsynthesis/speech-synthesis-voices.html,virtual/stable/webexposed/global-constructors-listing.html,webexposed/global-constructors-listing.html



Original issue's description:
> Disable speech synthesis API for content/ layer.
> 
> The Speech Synthesis API relies on an implementation in chrome/
> yet is a stable Web Platform feature so enabled in the content/
> layer. Flip the enable flag to a disable flag, and set it for
> non-chrome content/ embedders.
> 
> BUG=347045
> 
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=280667

TBR=dmazzoni@chromium.org,brettw@chromium.org,benm@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=347045
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=280749

[pfeldman, 2014-07-01 05:02:49]

Created Revert of Disable speech synthesis API for content/ layer.

[commit-bot: I haz the power, 2014-07-01 05:03:26]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/pfeldman@chromium.org/359413005/1

[commit-bot: I haz the power, 2014-07-01 05:04:31]

Change committed as 280749