Change unit for tab_capture_performancetests to ms.

BUG=336028

Review URL: https://codereview.chromium.org/144343003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247479 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247479