Convert OVERRIDE -> override and update virtual/final usage in tools/gn/.

Update virtual/final usage in tools/gn/.

Also some #include fixes (from OVERRIDE -> override conversion).

BUG=417463
TBR=brettw@chromium.org

Committed: https://chromium.googlesource.com/chromium/src/+/50df3edc46d054d4b7a6cec4098cd1538d66d511

[Peter Kasting, 2014-10-21 22:48:09]

This seems to overlap with https://codereview.chromium.org/667143003/ .

[viettrungluu, 2014-10-22 02:28:30]

Stealing the LGTM from dcheng's change and landing this TBR (for the extra
OVERRIDE -> override changes).

[viettrungluu, 2014-10-22 04:24:28]

On 2014/10/22 02:28:30, viettrungluu wrote:
> Stealing the LGTM from dcheng's change and landing this TBR (for the extra
> OVERRIDE -> override changes).

Ha, someone else got to the OVERRIDEs first. Still gonna land this, to spare
dcheng the pain of updating his change (and to keep my #include fixes).

[viettrungluu, 2014-10-22 04:24:34]

The CQ bit was checked by viettrungluu@chromium.org

[commit-bot: I haz the power, 2014-10-22 04:25:41]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/643063004/20001

[commit-bot: I haz the power, 2014-10-22 05:05:53]

Patchset 2 (id:??) landed as
https://crrev.com/50df3edc46d054d4b7a6cec4098cd1538d66d511
Cr-Commit-Position: refs/heads/master@{#300635}

[viettrungluu, 2014-10-22 05:08:37]

Committed patchset #2 (id:20001) manually as
50df3edc46d054d4b7a6cec4098cd1538d66d511 (presubmit successful).