Clipboard: Remove unnecessary CHECK

- This CHECK never crashed, so turn it into a DCHECK.

Bug: 918717
Change-Id: Icb9d4fb43f20c413014c4519ff49f42182df3035
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1497195
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Darwin Huang <huangdarwin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637505}
1 file changed