Fixed copyright message in AOMContentAXTree by removing the (C) symbol

Chromium's new copyright message doesn't have a (C) symbol.
Files created in the last few years should be using the new copyright message.
TBR=dmazzoni@chromium.org, aleventhal@chromium.org

Change-Id: I653ea93d569c8eb21810007dad9df1021d9cf532
Reviewed-on: https://chromium-review.googlesource.com/c/1467024
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631306}
2 files changed