Telemetry: add multitab tests framework.

Adds to Telemetry the ability to map pages to tabs instead of having
all pages open sequentially in a single tab.

BUG=chromium:249906
TEST=ran the modified memory_multi_tab test

Review URL: https://codereview.chromium.org/26705004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229623 0039d316-1c4b-4281-b951-d872f2087c98
Move VideoDecoderVp8 to VideoDecoderVpx base that will support VP8/9.

BUG=260879

Review URL: https://codereview.chromium.org/26888009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229624 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229624