Revert of Revert "Typecheck JS files for chrome://extensions"

Revert of Revert "Typecheck JS files for chrome://extensions" (patchset #1 of https://codereview.chromium.org/508283002/)

Reason for revert:
fixed check_licenses issues

Original issue's description:
> Revert "Typecheck JS files for chrome://extensions"
> 
> This reverts commit 405253b7ee22ecb318d3c1ca9187e1e7259e704f.
> 
> Caused a liscene failure on android bot:
> http://build.chromium.org/p/chromium.linux/builders/Android%20Builder%20%28dbg%29/builds/64664/steps/check_licenses/logs/stdio
> 
> Committed to pending queue: https://chromium.googlesource.com/chromium/src/+/87b3206

TBR=vitalyp@chromium.org,eroman@chromium.org
NOTREECHECKS=true
NOTRY=true

Committed: https://crrev.com/9a5ab1949738fac262adbcf125004f7b1f62e7e9
Cr-Commit-Position: refs/heads/master@{#292229}

[Dan Beam, 2014-08-27 21:25:40]

Created Revert of Revert "Typecheck JS files for chrome://extensions"

[commit-bot: I haz the power, 2014-08-27 21:27:12]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/dbeam@chromium.org/512003002/1

[commit-bot: I haz the power, 2014-08-27 21:30:39]

Committed patchset #1 (id:1) as 9322608cce3fd8f52af39613bdaf52fac4ea3398

[commit-bot: I haz the power, 2014-09-10 02:54:15]

Patchset 1 (id:??) landed as
https://crrev.com/9a5ab1949738fac262adbcf125004f7b1f62e7e9
Cr-Commit-Position: refs/heads/master@{#292229}