Pepper: Constify a BrowserPpapiHostImpl function.

BrowserPpapiHostImpl::external_plugin() doesn't mutate anything and should be
const. Making this small change makes moving FileIO from the renderer to the
browser cleaner.

BUG=246396
R=bbudge@chromium.org

Review URL: https://codereview.chromium.org/28053004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229439 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229439
Jumping through hoops because blink depends on content...

This change just prepares moving content shell and content tests to
their own gyp file.

First, I'll introduce a temporary target for blink (this is this CL).
Then, I'll update blink to the temporary target. This will allow for moving content shell and content tests to their final destination.
The last step is to update blink once more to use the correct new target.

R=jam@chromium.org

Review URL: https://codereview.chromium.org/28203002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229440 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229440