Create EmbeddedTestServer's thread when necessary

BUG=321422

Review URL: https://codereview.chromium.org/76983005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236422 0039d316-1c4b-4281-b951-d872f2087c98
CC: Handle the case where high-res is the previous-low-res tiling.

Otherwise we can end up with no high-res tiling, as high-res
is set to LOW_RESOLUTION

Using 2X sizes exercises this code path more often, but this
looks correct to do in any case.

TBR=enne@chromium.org
BUG=321523, 307206

Review URL: https://codereview.chromium.org/77043012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236423 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236423