Fix leak of 74,000 BidiCharacterRun objects in BidiResolver.BidiTest_txt

BidiResolver has a bad API whereby callers are expected to manually
delete the runs it creates.

There is a FIXME in BidiRunList.h:
    // FIXME: Once BidiResolver no longer owns the BidiRunList,
    // then ~BidiRunList should call deleteRuns() automatically.
But until that's resolved, we need this deleteRuns() call
to avoid leaking every BidiCharacterRun this test creates.

This was caught by the RefCountedLeakCounter.

Review URL: https://codereview.chromium.org/282563002

git-svn-id: svn://svn.chromium.org/blink/trunk@173880 bbb929c8-8fbe-4397-9dbb-9b2b20218538