PPAPI: Generalize PPB_Ext_CrxFileSystem as PPB_IsolatedFileSystem

This change generalizes PPB_Ext_CrxFileSystem_Private, built on the isolated
filesystem, so that we can easily reuse its components when introducing new
isolated filesystem in PPAPI.

This adds PPB_IsolatedFileSystem_Private API and enum indicating isolated
filesystem type. PPB_Ext_CrxFileSystem_Private API is still remaining since
some plugins may use the API. Both API share pepper resource and host
implementations.


BUG=286242
TEST=manual (see [1] and [2])
TBR=cpu@chromium.org

[1] https://code.google.com/p/chromium/issues/detail?id=271126#c7
[2] https://codereview.chromium.org/59203002/

Review URL: https://codereview.chromium.org/51653004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233807 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233807