[fsp] Extract common code for metadata handling in browser tests.
Because most of the browser tests of the File System Provider API need some
testing files to work on, there was a lot of duplicated code.
This patch extracts the metadata handling code as a default implementation
of the onMetadataRequested event.
TEST=browser_tests: *FileSystemProvider*
BUG=248427
Review URL: https://codereview.chromium.org/
373743002
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@281644 0039d316-1c4b-4281-b951-d872f2087c98