linux-aura: Special case text-types when pasting from clipboard.

Some apps (e.g. firefox) copies plain text content in the clipboard with selection
types TEXT, STRING etc., but not text/plain (which chrome expects). So special case
text content in the clipboard, and compare the type against a known number of text-
types.

BUG=317514
R=dcheng@chromium.org, erg@chromium.org

Review URL: https://codereview.chromium.org/135593002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244569 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244569