Multivm branch: Fetch reference build from archive using git hash, not revision.

deps/multivm.deps/DEPS

 import gclient_utils
 import os
 
 path = gclient_utils.FindGclientRoot(os.getcwd())
 execfile(os.path.join(path, 'multivm.deps', 'DEPS.chromium')) # Include proper Chromium DEPS.
 
 # Now we need to override some settings and add some new ones.
 
 vars.update({
   "chromium_url": "http://src.chromium.org/svn",
   "multivm_base": "http://src.chromium.org",
   "multivm_chromium_branch": "/chrome/trunk/src",
   "multivm_chromium_revision": "278856",
-  # There were no uploaded chromium builds within 100 revisions of 277580.
-  "chromium_base_revision": "278856",
+  # Git hashes of snapshotted chromium builds closest to r278856.
+  "chromium_base_revision":
+      '{"Windows":"37fde6a752162681ce6b62f1a96683483f2df596", ' +
+      ' "Darwin": "db25a7e9bf34e638a1cbb4c8974ca566c22eb62c", ' +
+      ' "Linux": "abe29e2ae1ad6d9bd8a7bfa23b3316a203d39ef2"}'
   "dart_tools_branch": "/branches/bleeding_edge/dart/tools",
   "dart_tools_revision": "36985",
   "multivm_blink_branch": "/blink/branches/dart/multivm",
   "multivm_blink_revision": "176873",
 })
 
 def massage_deps(deps):
   for key, value in deps.items():
     if value is None: continue
 
@@ skipping 38 matching lines @@
 
 hooks.append({
   # Set the revision for a lazily downloaded reference build of chromium,
   # with which to run perf tests.
   'name': 'set_reference_build',
   'pattern': '.',
   'action': ['python',
              'src/dart/tools/bots/set_reference_build_revision.py',
              Var('chromium_base_revision')],
 })