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At this moment, the texImage2D and texSubImage2D for ImageBitmap is still
hiding behind a RuntimeEnabled flag. Since ImageBitmap has been shipped:
https://codereview.chromium.org/1636633002/
We should remove this run time flag as well.

Review URL: https://codereview.chromium.org/1632853003

Cr-Commit-Position: refs/heads/master@{#371569}