Add libva generator.

Add libva generator.

BUG=None
R=cmasone@chromium.org
TBR=jln@chromium.org
TEST=comparing generated libva files between gyp and gn

Committed: https://crrev.com/f7571f31090438a7f5474efcd1a72983b656e97b
Cr-Commit-Position: refs/heads/master@{#297477}

[Jun Mukai, 2014-09-30 00:42:05]

mukai@chromium.org changed reviewers:
+ jln@chromium.org

[Chris Masone, 2014-09-30 01:02:44]

The action definition looks fine to me GN-wise

in content_common.gypi, though, the rule that does this generation also adds a
dependency on ../media/media.gyp:media and libyuv. Should that be replicated
here?

[Jun Mukai, 2014-09-30 02:30:22]

oops, haven't noticed that.  Updated.
note that "<(output_root)" won't be necessary in GN because of output file path.

[Chris Masone, 2014-09-30 03:28:45]

lgtm

[Jun Mukai, 2014-09-30 18:07:33]

let me go with TBR=jln because his status says he's slow right now, and this CL
only modifies GN files which got an lgtm from a GN guy.

[Jun Mukai, 2014-09-30 18:07:51]

The CQ bit was checked by mukai@chromium.org

[commit-bot: I haz the power, 2014-09-30 18:09:07]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/601923005/20001

[commit-bot: I haz the power, 2014-09-30 19:19:18]

Committed patchset #2 (id:20001) as b2f60d589fef217eda67b5e53cf717db57e5b01b

[commit-bot: I haz the power, 2014-09-30 19:20:11]

Patchset 2 (id:??) landed as
https://crrev.com/f7571f31090438a7f5474efcd1a72983b656e97b
Cr-Commit-Position: refs/heads/master@{#297477}