Revert 191420 "PPAPI: Remove threading options; it's always on"

> PPAPI: Remove threading options; it's always on
> 
> This also re-enables thread checking for the host side resource and var trackers. Before, checking was disabled everywhere.
> 
> BUG=159240,92909
> 
> 
> Committed: r186925
> Reverted: r186939 due to build errors
> 
> Committed: r187340
> 
> Committed: r187427
> Reverted: r187668 due to a failing check in Canary, which was fixed here: r187681
> 
> Committed: r189518
> Reverted: r189682, due to regression in Kraken (see crbug.com/222741)
> 
> Review URL: https://chromiumcodereview.appspot.com/12378050

TBR=dmichael@chromium.org
Review URL: https://codereview.chromium.org/13328006

git-svn-id: http://src.chromium.org/svn/trunk/src/build@191441 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

common.gypi

@@ -322,6 +322,11 @@
       # Webrtc compilation is enabled by default. Set to 0 to disable.
       'enable_webrtc%': 1,
 
+      # PPAPI by default does not support plugins making calls off the main
+      # thread. Set to 1 to turn on experimental support for out-of-process
+      # plugins to make call of the main thread.
+      'enable_pepper_threading%': 1,
+
       # Enables use of the session service, which is enabled by default.
       # Support for disabling depends on the platform.
       'enable_session_service%': 1,
@@ -678,6 +683,7 @@
     'use_x11%': '<(use_x11)',
     'use_gnome_keyring%': '<(use_gnome_keyring)',
     'linux_fpic%': '<(linux_fpic)',
+    'enable_pepper_threading%': '<(enable_pepper_threading)',
     'chromeos%': '<(chromeos)',
     'enable_viewport%': '<(enable_viewport)',
     'enable_hidpi%': '<(enable_hidpi)',
@@ -1781,6 +1787,9 @@
       ['proprietary_codecs==1', {
         'defines': ['USE_PROPRIETARY_CODECS'],
       }],
+      ['enable_pepper_threading==1', {
+        'defines': ['ENABLE_PEPPER_THREADING'],
+      }],
       ['enable_viewport==1', {
         'defines': ['ENABLE_VIEWPORT'],
       }],