Explicit GC of strings to avoid test timeout.

The test file visited-link-hang.html creates millions of strings with length
1-1000000. That caused the giant-stylesheet-crash.html test to time out if
the strings were still not garbage collected. Added explicit garbage
collection to clean up after the visited-link-hang strings. Likewise, added
explicit GC calls as a safety measure in the test that timed out.

R=ojan@chromium.org
BUG=305037

Review URL: https://codereview.chromium.org/38293002

git-svn-id: svn://svn.chromium.org/blink/trunk@160683 bbb929c8-8fbe-4397-9dbb-9b2b20218538