Relanding this with fixes to the mac dbg builder

Relanding this with fixes to the mac dbg builder

Remove the Webkit thread in the PPAPI plugin process and perform the text and font operations
on the PPAPI main thread. This thread is now registered as the Webkit thread.

Fixes performance issues seen in Flapper with text and font operations. It appears that the perf
issues occur due to context switching between the main thread and the webkit thread.

As per comments from Brett moving the font forwarding code inline to ppb_font_shared.cc. This file
has been moved to ppapi/shared_impl/private as it now brings in a dependency on WebKit. The font
creation has been wired up to the ResourceCreationAPI as suggested.

BUG=110190
TBR=brettw
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=118503