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 "Allow this site to use your microphone"
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 that disabling this does not affect the new
JS Web Speech API by testing with:
https://www.google.com/intl/en/chrome/demos/speech.html

BUG=360448

Review URL: https://codereview.chromium.org/231323003

git-svn-id: svn://svn.chromium.org/blink/trunk@171187 bbb929c8-8fbe-4397-9dbb-9b2b20218538