Mojo: Replace TestWithIOThreadBase with just a helper TestIOThread class.

This makes "adding" an I/O thread easier (and usable when you already
have a test base class).

R=sky@chromium.org

Review URL: https://codereview.chromium.org/187383007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255196 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255196
Disable failing tests for WV key system running with test license server.

crbug/349398: Disabled Widevine test license server on precise 32 bit.
crbug/349526: Disabled two failing tests on precise 64 bit.

BUG=349526,349398

Review URL: https://codereview.chromium.org/180733004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255198 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255198