Oilpan: move 2D Canvas and WebGL objects to the heap.

This adds Oilpan transition types to the WebGL object hierarchy, along
with some remaining Canvas 2D objects. 

Notes:

- Oilpan does not allow destructors to access other garbage collected
  objects and we want all WebGL script accessible objects to be under
  the control of Oilpan and be garbage collected. To follow and
  fulfill both, we cannot make the objects that the destructors depend
  on for fetching out the graphics context (which they use to finalize
  their platform resources) be garbage collected. For
  WebGLSharedObjects (and WebGLRenderingContextBase) that means that
  WebGLContextGroup cannot be on the heap/garbage-collected, and for
  WebGLContextObjects, the WebGLRenderingContextBase's DrawingBuffer
  cannot be on the heap either.
  
  See details below for how this is done for WebGLContextObject's; the
  changes to WebGLContextGroup and WebGLSharedObject is along similar
  lines.
  
- In terms of file footprint of the changes here, the WebGLExtension-derived
  objects make up a significant portion. Their switch in transition
  types for their create() constructor method is all standard and
  repetitive.

- The release of platform resources when finalizing WebGL objects
  requires some extra care with Oilpan. That release is done using a
  platform graphics context object (blink::WebGraphicsContext3D), which
  is owned by a DrawingBuffer. As finalization order isn't deterministic
  with Oilpan, we have to arrange for that DrawingBuffer's context
  object to remain alive until all objects that need it for platform
  resource release have been finalized. And we cannot rely on fetching
  the context object from another WebGL object on the Oilpan heap, as it
  may or may not have been finalized.

  To address, the (off heap) DrawingBuffer is shared among all objects 
  that need it for finalization, using an off-heap RefCounted abstraction 
  (WebGLSharedWebGraphicsContext3D). When all objects have been finalized, 
  that shared object will be destructed and its underlying DrawingBuffer 
  (and, indirectly, its blink::WebGraphicsContext3D object) is let go of. 

  As a result, each WebGLContextObject adds a RefPtr<> to this shared 
  DrawingBuffer/WebGraphicsContext3D with Oilpan enabled. 

R=haraken@chromium.org,ager@chromium.org,junov@chromium.org,bajones@chromium.org,kbr@chromium.org
BUG=340522

Review URL: https://codereview.chromium.org/365653002

git-svn-id: svn://svn.chromium.org/blink/trunk@178647 bbb929c8-8fbe-4397-9dbb-9b2b20218538