This adds SkResourceCache::Remove() which will remove a resource from
its cache. The resource is required to be unlocked at the time Remove()
is called.
Then SkBitmapCache::Find() makes use of this to Remove() bitmaps from
the cache whose pixels have been evicted. This allows the bitmap to be
re-added to the cache with pixels again.
After this change, background a tab (and discarding all the bitmaps'
contents) no longer disables image caching for those discarded images
once the tab is visible again.
BUG=skia:2926
NOTRY=true
R=reed@android.com, tomhudson@google.com, reed@google.com
Author: danakj@chromium.org
Review URL: https://codereview.chromium.org/561953002
