Add Docs for chromeexts

BUG=1168231

Change-Id: I53576e90421502a90522f71c0bcd2b027006de60
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2725316
Auto-Submit: Robert Liao <robliao@chromium.org>
Commit-Queue: Wei Li <weili@chromium.org>
Reviewed-by: Wei Li <weili@chromium.org>
Cr-Commit-Position: refs/heads/master@{#858609}
GitOrigin-RevId: 0329f3957a6fbf5fc96e1c8c588312c505a76628
1 file changed