[Android WebView] Fix outstanding license check issues on the release bot

[Android WebView] Fix outstanding license check issues on the release bot

 - fix missing files check for the case when the script runs outside of
   the repository root;

 - exclude not shipped dirs that present on the bot from the checks.

BUG=450048
Committed: https://crrev.com/da58b9ae8fabdd2047cdea05bc749e8697edeed2
Cr-Commit-Position: refs/heads/master@{#314310}

[mnaganov (inactive), 2015-02-02 18:39:44]

mnaganov@chromium.org changed reviewers:
+ torne@chromium.org

[Torne, 2015-02-03 10:29:25]

lgtm

[mnaganov (inactive), 2015-02-03 10:35:41]

The CQ bit was checked by mnaganov@chromium.org

[commit-bot: I haz the power, 2015-02-03 10:35:51]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/877583003/1

[commit-bot: I haz the power, 2015-02-03 11:09:27]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-03 11:10:58]

Patchset 1 (id:??) landed as
https://crrev.com/da58b9ae8fabdd2047cdea05bc749e8697edeed2
Cr-Commit-Position: refs/heads/master@{#314310}