Make GN header checker more lenient about toolchains.

Make GN header checker more lenient about toolchains.

The header checker will now automatically accept any headers not known in the
current toolchain. The previous code would do this but required it to not be
present in any toolchain in the entire build.

See the long comment added in header_checker.cc for details.

Adds a dependency fix for the Android build in base.

BUG=

Committed: https://crrev.com/a9adafeae2a97bc5a9676b4072ddf8491c212c4d
Cr-Commit-Position: refs/heads/master@{#330839}

[brettw, 2015-05-20 22:17:03]

brettw@chromium.org changed reviewers:
+ scottmg@chromium.org

[brettw, 2015-05-20 22:17:09]

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

[commit-bot: I haz the power, 2015-05-20 22:18:08]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1142423004/20001

[scottmg, 2015-05-20 22:26:16]

lgtm

[brettw, 2015-05-20 22:28:30]

The CQ bit was unchecked by brettw@chromium.org

[brettw, 2015-05-20 22:28:33]

The CQ bit was checked by brettw@chromium.org

[commit-bot: I haz the power, 2015-05-20 22:29:13]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1142423004/20001

[commit-bot: I haz the power, 2015-05-20 23:32:46]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-05-20 23:34:31]

Patchset 2 (id:??) landed as
https://crrev.com/a9adafeae2a97bc5a9676b4072ddf8491c212c4d
Cr-Commit-Position: refs/heads/master@{#330839}