Do not ignore src/third_party/cros_system_api

This is now a dependency for the Storage Monitor. It may sound like it is
ChromeOS-only dependency, but the Media Transfer Protocol needs a file from it.
DEPS.xwalk: Roll chromium-crosswalk to 655d52b8

655d52b8 Merge pull request #117 from tmpsantos/storage_monitor
0fb3de5 [SysApps] Compile StorageMonitor as a component
[SysApps] Add a data provider for Linux based on Chromium's StorageMonitor

This provider has a big chance of being generic enough to be used for Mac and
Windows. The backend for Android was not implemented yet which makes me sad.

We will have to implement a custom provider for Android (upstream maybe?).