Fix Dr Memory download

In http://crrev.com/275232 the drmemory.DEPS directory was removed
since the Chromium bots have moved over to download from Google
Storage (http://crrev.com/275048).
This CL changes WebRTC to use the same approach.

Ideally the revision for the Dr Memory DEPS entry should use the
chromium_revision variable, but when I tried to roll to that revision
in https://review.webrtc.org/19679004/ I ran into errors with leaks
being detected in the compile step on the Linux ASan bot.
This CL allows our Dr Memory bots to go green while investigating this.

BUG=chromium:381366
TEST=Passing Win Dr Memory trybots.
R=tommi@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/13619004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6366 4adac7df-926f-26a2-2b94-8c16560cd09d