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Usually the clipboard history menu is anchored at the caret's location.
This CL anchors the history menu at the cursor's location when there is
no textfield. In addition, some apps may provide unreliable caret's
bounds, such as Google Doc. The menu is also anchored at the cursor's
location in that case. Subsequent CLs will solve the unreliable caret
bounds thoroughly.

A browser test is created to protect this feature.

Bug: 1110027
Change-Id: I30f31b0ab89542b5c7f63ebd25257dc765f57929
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2328191
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#794244}