Avoid touching ResourceBundle fonts from non-UI thread

SharedResourcesDataSource touches shared fonts in ResourceBundle from
IO thread. However, gfx::Font is not thread safe due to non-thread-safe
ref counts in gfx::PlatformFont and gfx::FontListImpl. That causes a
data race on the ref count.
So, we should touch fonts in ResourceBundle only from UI thread.

BUG=688072, 468010

Review-Url: https://codereview.chromium.org/2701183002
Cr-Commit-Position: refs/heads/master@{#452390}
1 file changed