WebGL: Restore atomically both WebGraphicsContext3D and DrawingBuffer together.

Currently, WebGLRenderingContextBase::maybeRestoreContext() has a potential crash
bug. When WebGraphicsContext3D is created but fbo of DrawingBuffer is not
created, WebGLRenderingContextBase considers it as success, although m_context
is not updated. It means that DrawingBuffer holds a dangling pointer.

This CL changes the success of restoration that completes both
WebGraphicsContext3D creation and fbo creation of DrawingBuffer.

BUG=344393

Review URL: https://codereview.chromium.org/189423003

git-svn-id: svn://svn.chromium.org/blink/trunk@170745 bbb929c8-8fbe-4397-9dbb-9b2b20218538