[PPAPI] Added pp::VarResource_Dev class.

This is a C++ wrapper for the C API PPB_VarResource_Dev.

Also added methods to pp::FileSystem for converting a pp::Resource to a
pp::FileSystem, which are necessary for making use of a resource
extracted using pp::VarResource_Dev.

(Committed by yzshen@chromium.org on behalf of mgiuca@chromium.org)

BUG=177017
R=dmichael@chromium.org, noelallen@chromium.org, noelallen@google.com, yzshen@chromium.org

Review URL: https://codereview.chromium.org/52233002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232482 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232482