Remove all the code for --run-singly from run-webkit-tests

The --run-singly flag only ever really made sense when we couldn't
count on the driver implementation (DRT/content_shell) to enforce
the timeout for a test. Once we implemented proper support in driver.run_test()
for enforcing timeouts directly (and made that a requirement of the interface,
--run-singly was just unnecessary overhead, and I don't think anyone really used
it.

This patch removes the code (and tests) and makes --run-singly a synonym for
--batch-size=1 --verbose (which basically has the same effect of restarting
the driver after every test).

R=ojan@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/22859045

git-svn-id: svn://svn.chromium.org/blink/trunk@156598 bbb929c8-8fbe-4397-9dbb-9b2b20218538