[Chromecast] Allow |enable_pepper_cdms| to be set in gn args.

[Chromecast] Allow |enable_pepper_cdms| to be set in gn args.

Chromecast builds need to explicitly set |enable_browser_cdms| to true
and |enable_pepper_cdms| to false. Rather than introduce a global
is_chromecast variable, as we have in GYP, allow |enable_pepper_cdms| to
be set in args.gn.

BUG= b/17615224

Committed: https://crrev.com/9bacb78d7d0a453ff5d23df7fd30b7b75cded17d
Cr-Commit-Position: refs/heads/master@{#341776}

[slan, 2015-08-04 17:23:56]

slan@chromium.org changed reviewers:
+ alokp@chromium.org, brettw@chromium.org, gunsch@chromium.org

[slan, 2015-08-04 17:23:56]

My goal is to avoid an "is_chromecast" variable. Here the corresponding GYP
change:
https://chromium.googlesource.com/chromium/src/+/e4513848535ec9ad29c40320ac2e...

PTAL.

[brettw, 2015-08-04 17:30:18]

lgtm

[slan, 2015-08-04 17:47:02]

The CQ bit was checked by slan@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-08-04 17:47:19]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1266693005/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1266693005/1

[commit-bot: I haz the power, 2015-08-04 18:50:06]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-08-04 18:50:06]

Dry run: This issue passed the CQ dry run.

[gunsch, 2015-08-04 19:43:37]

lgtm

[slan, 2015-08-04 20:05:14]

The CQ bit was checked by slan@chromium.org

[commit-bot: I haz the power, 2015-08-04 20:05:58]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1266693005/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1266693005/1

[commit-bot: I haz the power, 2015-08-04 20:13:03]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-04 20:13:38]

Patchset 1 (id:??) landed as
https://crrev.com/9bacb78d7d0a453ff5d23df7fd30b7b75cded17d
Cr-Commit-Position: refs/heads/master@{#341776}