Allow RenderWidgetHostViewMac::SyncGetFirstRectForRange to block.

This lets -[RenderWidgetHostViewCocoa
firstRectForCharacterRange:actualRange:] block, which currently DCHECKs
when the requested range isn't in the active selection.

Bug: 42434, 121917
Change-Id: I905650b33d065d68ceb13c1a73a74efcb41ecffa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1614702
Reviewed-by: Mark Mentovai <mark@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Sidney San Martín <sdy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661482}
3 files changed