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.

Change-Id: I50e9321babaaf039e25e30f7ead6527169d32e1b

Conflicts:
	src/com/android/browser/BrowserSettings.java