[fsp] Cleanup browser tests.

This patch adds jsdocs, and missing chrome.test.callbackPass invocations.

Moreover, it makes the api methods return true in case of error. It was wrong
to cause an immediate error together with returning DOMError. The immediate
error should be only returned in case of wrong argument, which is checked
by the EXTENSION_FUNCTION_VALIDATE macro.

TEST=browser_tests: *FileSystemProvider*
BUG=248427

Review URL: https://codereview.chromium.org/268573003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@267508 0039d316-1c4b-4281-b951-d872f2087c98
3 files changed