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With the new algorithm in http://crrev/c/1102157, we won't be able to
select cross editing boundary anymore, so layout test
search-cancel-button-mouseup.html will violate a DCHECK in
|TextControlElement::CacheSelection()|.

In fact, we shouldn't allow selection starts with cancel button at all,
this CL adds |user-select: none;| to |SearchFieldCancelButtonElement|.

Bug: 863651
Change-Id: Id23b50503caaf814b0a8a14ea1b7c243b3c51c68
Reviewed-on: https://chromium-review.googlesource.com/1141350Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Shimi Zhang <ctzsm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576321}