fileSystemProvider: Add a namespace-level comment

Lack of the comment caused a log spam in the build like:

[2/41] RULE Generating C++ code from file_system_provider.idl IDL files
fileSystemProvider must have a namespace-level comment. This will appear on the API summary page.

BUG=317419
TEST=the log spam is gone from the build log
R=benwells@chromium.org

Review URL: https://codereview.chromium.org/68273002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234193 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234193