Implement HTMLCanvasElement's transferControlToOffscreen
To summarize, this function throws an exception when the canvas has an
associated context. A new OffscreenCanvas is created and it has a pointer
pointing to the originated canvas element.
A layout test is also added to make sure that there is no exception when
the original canvas doesn't have a context, and throws an exception when
the canvas does have a context.
BUG=598753
Review URL: https://codereview.chromium.org/1836283002
Cr-Commit-Position: refs/heads/master@{#384627}
11 files changed
