PPAPI: Add PluginPrivateFileSystem

This change introduces PluginPrivate fileSystem, a brand-new per-plugin
sandboxed-isolated filesystem, in PPB_IsolatedFileSystem_Private.

Key points in this CL:
- Adding new isolated filesystem type for PluginPrivate filesystem.
- Granting full access of the filesystem to renderer process.
- Generating plugin ID from plugin's MIME type.


BUG=286242
TEST=manual (see https://codereview.chromium.org/77813004/)
TEST=content_unittests --gtest_filter=PepperFileSystemBrowserHostTest.*
TBR=jochen@chromium.org

Review URL: https://codereview.chromium.org/26803004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236241 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236241