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Before, the option would appear regardless of the default search engine,
but would DCHECK if the default search engine didn't support search-by-
image. Now, the option doesn't appear in that case.

Bug: 935386
Change-Id: Ic1e1ef896674b72bd8de8f159cf8afb9718c93a5
Reviewed-on: https://chromium-review.googlesource.com/c/1486093
Commit-Queue: Robbie Gibson <rkgibson@google.com>
Reviewed-by: David Roger <droger@chromium.org>
Reviewed-by: Stepan Khapugin <stkhapugin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636751}