Stocks.apk should have icons when run offline

Stocks.apk should have icons when run offline

This CL adds the concept of a root asset bundle to Sky. If the Sky app was
launched from a bundle, the root bundle is that bundle.

This CL teaches icon.dart to default to loading icons out of the root bundle,
if a root bundle exists. The next result is that Stocks has icons when run
offline because the icons are present in stock's skyx bundle.

TBR=eseidel@google.com

Committed: https://chromium.googlesource.com/external/mojo/+/71f1f5ba310c7a13ace30b1c1a66aefecc381e02

[abarth-chromium, 2015-07-02 20:47:18]

Committed patchset #1 (id:1) manually as
71f1f5ba310c7a13ace30b1c1a66aefecc381e02 (presubmit successful).