Update sanitizer-related code in the test runner.

- add support for MemorySanitizer,
- asan_symbolize.py now demangles by default, no need to use c++filt,
- pass strip_path_prefix to asan_symbolize.py to relativize the source paths,
- fix an error in comment.

BUG=chromium:397119
R=dpranke@chromium.org
TEST=MSan

Review URL: https://codereview.chromium.org/412223002

git-svn-id: svn://svn.chromium.org/blink/trunk@179299 bbb929c8-8fbe-4397-9dbb-9b2b20218538