POSIX: Use Shared Mem transport to copy images.

      Prior to this change images where copied inline in IPC messages on non-Windows platforms.  Copying an oversized image would cause the IPC system to bork and crash the renderer.

Changes in this CL:
* All platforms use a unified mechanism to copy images using shared memory.
* Introduced a new IPC message so the renderer can allocated a shared memory segment on OS X.
* On OS X tried to keep as few copies of the image data in memory as possible.

BUG=26822
TEST=1)On all platforms: navigate to a webpage, right click on an image and copy.  Then try pasting into an image editor. 2)Repro steps in bug should no longer crash the Renderer on Mac/Linux

Review URL: http://codereview.chromium.org/552129

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@37247 0039d316-1c4b-4281-b951-d872f2087c98