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This limits users from entering text either by typing
or by pasting text that is beyond a max of 300-char length.

BUG=1067418
TEST=Manually, added a new test.

Change-Id: I3e4e5d9cba52be98fef3486c0fa5297ad9afb324
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2129229Reviewed-by: Jun Mukai <mukai@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756328}