Reland: Cleanup: Remove extra EmbeddedWorker layering in the renderer SW code
Original review: https://codereview.chromium.org/
1136323003/
Threading fixes from the original patch (PS1):
- Don't bind WebString (as it's not thread-safe)
- Introduce internal WorkerContextData struct for holding data
(specifically callback maps) that need to be bound/destructed on the
worker thread.
BUG=491039, 492625, 492570
TEST=existing tests, layout tests
Review URL: https://codereview.chromium.org/
1157273009
Cr-Commit-Position: refs/heads/master@{#333437}