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This matches the behavior of Cocoa textfields, and also makes it possible to
right-click paste at the end of a textfield.

Bug: 856609
Change-Id: Ic609b2bb78737d379e7e0a57ab3f4c7f16a3cec9
Reviewed-on: https://chromium-review.googlesource.com/1160768
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580260}