[fsp] Simplify browser tests by extracting the common code.

This patch extracts the common code used by most of File System Provider API
browser tests to a helper extension.

Note, that this is the first cleanup step. The next one would be creating
a utility function for creating fake metadata (since it takes too much of each
test).

TEST=browser_tests: *FileSystemProvider*
BUG=248427

Review URL: https://codereview.chromium.org/374543002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@281547 0039d316-1c4b-4281-b951-d872f2087c98
18 files changed