Notify V8 when we have many WebGL contexts

This fix is to deal with the WebGL 1.0.2
context-creation-and-destruction test which creates 500 contexts quickly
and relies on JS cleaning up contexts as they go out of scope. WebGL
context objects consume almost no memory on the JS heap, and V8 does not
know what kind of external resources are associated with them. The
garbage collector is likely not running frequently enough to dispose of
the contexts. Calling lowMemoryNotification when we have many contexts
should force a major GC and free up the WebGL resources.

Change-Id: I3c8f51ddeede899e2b6a3d14a87b8c80883dd498
CRs-fixed: 427617
Disable OES_texture_float WebGL extension

The following WebGL 1.0.2 conformance tests are causing crashes in
WebGLRenderingContext::texImage2DImpl:

conformance/extensions/oes-texture-float-with-canvas.html
conformance/extensions/oes-texture-float-with-image-data.html
conformance/extensions/oes-texture-float-with-image.html

Turning off the extension to avoid crashing.

Change-Id: Ideaf118cbd4770cd3b1f677bf342d9cc802a6688
CRs-fixed: 427617
Use combined depth and stencil buffers

Use combined depth and stencil buffers when both are requested together.
When stencil and depth buffers are requested, create a packed buffer
instead of standalone buffers and attach it to both the DEPTH_ATTACHMENT
point as well as the STENCIL_ATTACHMENT point as per
OES_packed_depth_stencil extension to GLES2.

CRs-fixed: 45920

Change-Id: I537efb611158935206c295fec8fc1a9b6d27e82b