Implement chrome.experimental.downloads.pause(), cancel() and resume().

BUG=12133
TEST=browser_tests --gtest_filter=DownloadsApiTest.*


Review URL: http://codereview.chromium.org/8517010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@114734 0039d316-1c4b-4281-b951-d872f2087c98
8 files changed