Re-implement --coverage in test-webkitpy to work properly.

Previously we would execute the coverage module directly after importing
the modules to run, so the top-level lines wouldn't be accounted for properly.
This patch changes the code to run coverage via the command line, so the
accounting is the same as if you were invoking coverage directly (and hence
is complete).

R=ojan@chromium.org, eseidel@chromium.org
BUG=none

Review URL: https://codereview.chromium.org/134643017

git-svn-id: svn://svn.chromium.org/blink/trunk@165345 bbb929c8-8fbe-4397-9dbb-9b2b20218538