Adds button label to extensions/shortcuts

The button for clearing an entered keyboard shortcut for extensions was
unlabeled, causing it to have no accessible name. This change adds the
Clear string to the grit resources.

BUG=870718

Change-Id: I19699be4b845db27a70d54b91ebdcdb108e71694
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1599766
Commit-Queue: Meredith Lane <meredithl@chromium.org>
Reviewed-by: Alice Boxhall <aboxhall@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659784}