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This CL checks if GetAssociatedFrameWidget() is valid before
GetStringAtPoint() since |blink_frame_widget_| from RenderWidgetHostImpl
could be unbound at some point. So, like other methods in
RenderWidgetHostImpl, TextInputClientMac::GetStringAtPoint() also check
GetAssociatedFrameWidget() is valid.

Bug: 1136471
Change-Id: Ia5a17e9ebf14bd0e3743b6236ddde0c08f0e4969
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2462921Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Julie Kim <jkim@igalia.com>
Cr-Commit-Position: refs/heads/master@{#816043}