Revert 171187 &quotDisable x-webkit-speech&quot

      http://build.chromium.org/p/chromium.webkit/builders/Mac10.6%20Tests/builds/24055
http://build.chromium.org/p/chromium.webkit/builders/Linux%20Tests/builds/35844

> Disable x-webkit-speech
> 
> This does not remove the code yet, only makes x-webkit-speech
> test-only (only exposed during LayoutTests) which makes
> this patch only a one-line change. We should follow up this
> change with a full removal of this feature including
> all tests.
> 
> x-webkit-speech is a Chrome-only feature which was
> announced as deprecated Feb 2014:
> https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/jlPdpx6jErE
> Current usage is at about 0.487% of reported Chrome page views
> which is (well) above our normal 0.06% removal threshold:
> http://www.chromestatus.com/metrics/feature/popularity#PrefixedSpeechAttribute
> 
> However, hours ago we recieved report of cases where *requiring
> user interaction* speech can be captured for up to 8 seconds
> or until the user stops un-interrupted speaking without proper
> appearance of the &quotAllow this site to use your microphone&quot
> infobar.
> 
> Although this is only a medium-security issue (requires
> user interaction and very limited duration), this is still
> severe enough that we no longer believe we can support
> this deprecated, webkit-only feature.
> 
> Removing this feature will not affect mobile users
> which already have access to speech-input via various
> speech-enabled keyboards on Android.  Desktop sites which
> were using this feature can use the modern Web Speech APIs
> instead, which are standardized and supported accross many
> browsers: http://caniuse.com/web-speech
> 
> Without this feature pages will continue to work, they just
> will not be able to get speech input via <input> controls
> via these APIs and will either have to use the standard
> Web Speech APIs or depend on the operating system to expose
> speech input via other methods.
> 
> I've verified t