Rolling v8/build to 38a9cfaa10bf2b91c52f8a9ef4449723dd7aa706

Rolling v8/tools/mb to 99788b8b516c44d7db25cfb68695bc234fdee5ed

TBR=machenbach@chromium.org,vogelheim@chromium.org,hablich@chromium.org

Review-Url: https://codereview.chromium.org/2267513002
Cr-Commit-Position: refs/heads/master@{#38765}