Runtime enablement of WebGL

Add Labs setting for enabling and disabling WebGL. Need to make sure
WebGL has been compiled in before showing runtime enablement settings in
Labs menu. Requesting information from the native side through JNI.

(cherry-picked commit from a2f7564cd95da42fad70e3c54497df7e7e8b1ea9)

Change-Id: Ie11bfa69941c2d75cfec04132908e07391c602fd