Add screen share icon to tab sharing infobar.

Update tab sharing infobar to display the screen share icon to the left,
as requested in the mocks linked in the bug.

Bug: 927949
Change-Id: I5c17f6b64e58b9893085e464ce300c55bc168152
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1651817
Commit-Queue: Marina Ciocea <marinaciocea@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#668753}