[fsp] Add the getMetadata operation.

This patch adds the getMetadata() operation, which basically returns basic
metadata of a file or a directory. It is tied to async file util's
getFileInfo() method.

TBR=isherman@chromium.org  # New histograms.
TEST=unit_tests, browser_test: *FileSystemProvider*GetMetadata
BUG=248427

Review URL: https://codereview.chromium.org/258783006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@267538 0039d316-1c4b-4281-b951-d872f2087c98
29 files changed