Expose an app_targets variable similar to android_app_targets

Expose an app_targets variable similar to android_app_targets

This target provides Chromium embedders with an elegant way to include
additional targets for all platforms.

BUG=453606
Committed: https://crrev.com/25c9c6c9b35147b4d3e9bababade01d9fe444968
Cr-Commit-Position: refs/heads/master@{#313831}

[Daniel Imms, 2015-01-29 22:34:56]

daniimms@amazon.com changed reviewers:
+ jochen@chromium.org

[Daniel Imms, 2015-01-29 22:36:20]

daniimms@amazon.com changed reviewers:
+ cjhopman@chromium.org

[cjhopman, 2015-01-29 22:40:25]

lgtm

[Daniel Imms, 2015-01-29 22:51:41]

The CQ bit was checked by daniimms@amazon.com

[commit-bot: I haz the power, 2015-01-29 22:52:36]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/882253007/1

[commit-bot: I haz the power, 2015-01-29 23:52:58]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-29 23:54:29]

Patchset 1 (id:??) landed as
https://crrev.com/25c9c6c9b35147b4d3e9bababade01d9fe444968
Cr-Commit-Position: refs/heads/master@{#313831}