Add GetSequenceNumber function to PPB_Flash_Clipboard

This provides access to a sequence number which identifies the clipboard state.

BUG=333094

Review URL: https://codereview.chromium.org/136183002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247383 0039d316-1c4b-4281-b951-d872f2087c98
15 files changed