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Remove FrameTreeNodeIdRegistry which is no longer needed now that networking callbacks are on the UI thread.

Change-Id: Ic66af3c207262aac8b955cb15a2f4a4922e0d898
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2524281
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825432}