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Previously, BidiCharacterRun used LayoutPartition, which is not thread
safe. GIven that now OffscreenCanvas can render text in a thread, those
runs need to be allocated/deleted from different threads.

Tested performance was neutral.

Bug: 799362
Change-Id: Id8766e23d4be1a41effeeb8956fa1b3e06dc49a3
Reviewed-on: https://chromium-review.googlesource.com/953331
Commit-Queue: Fernando Serboncini <fserb@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541858}