Fix thread checking logic in ContextProviderCommandBuffer

The thread check error addressed by https://codereview.chromium.org/1450433002
Was the result of an unnecessary thread check before thread attachment, which
was causing the thread checker to initialize itself on the wrong thread.

This CL reverts https://codereview.chromium.org/1450433002 and removes thread
checks from all the places where there were no thread checks prior to
https://codereview.chromium.org/1414683003/

BUG=556725
R=piman@chromium.org

Review URL: https://codereview.chromium.org/1459193003

Cr-Commit-Position: refs/heads/master@{#360928}
1 file changed