Re-land "WebGL: Transfer ownership of WebGraphicsContext3D to DrawingBuffer"

r168481 reverted r168457 "WebGL: Transfer ownership of WebGraphicsContext3D...",
which caused assertion failures on debug GPU bots. This CL relands the CL.

BUG=344393

Review URL: https://codereview.chromium.org/223223003

git-svn-id: svn://svn.chromium.org/blink/trunk@170970 bbb929c8-8fbe-4397-9dbb-9b2b20218538