Unshard WebKit Win7 dbg bots.

With the recent speed increases in debug builds of content_shell,
and the move to a new VM on the 720xd's, there's not really a good
reason to run the debug bots sharded any more. In the future,
if we need to shard tests across multiple machines, we'll want to
use swarming and isolates anyway.

TBR=ojan@chromium.org
BUG=270323

Review URL: https://codereview.chromium.org/23143003

git-svn-id: svn://svn.chromium.org/blink/trunk@156059 bbb929c8-8fbe-4397-9dbb-9b2b20218538