- Use valgrind from v8/third_party/valgrind fetched as custom
deps. This will make the check more portable on buildbot
slaves without the need to install valgrind.
- Build path to checked executable to be ready for swarming.

BUG=chromium:535160
LOG=n
NOTRY=true

Review URL: https://codereview.chromium.org/1585093002

Cr-Commit-Position: refs/heads/master@{#33287}