Implement Clipboard::ReadImage on Linux.

      I split the IPC up into two parts on Linux--one that reads the image on the UI
thread, and a second part that encodes the image on the file thread before
replying. I've also switched it to send the data over shared memory instead of passing a (potentially) huge blob of data over IPC.

BUG=75237
TEST=Local testing.

Review URL: http://codereview.chromium.org/6871001

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@82212 0039d316-1c4b-4281-b951-d872f2087c98