Instrumented libraries: add a hook to download pre-built binaries.

DEPS

Index: DEPS
diff --git a/DEPS b/DEPS
index 6350bae2be46abeb33d27da7a4b437c1699066d1..d943c1e92a919ffd1ae64ac91f9b70c774f014fc 100644
--- a/DEPS
+++ b/DEPS
@@ -758,6 +758,13 @@ hooks = [
               ],
   },
   {
+    # Pull sanitizer-instrumented third-party libraries if requested via
+    # GYP_DEFINES.
+    'name': 'instrumented_libraries',
+    'pattern': '\\.sha1',
+    'action': ['python', 'src/third_party/instrumented_libraries/scripts/download_binaries.py'],
+  },
+  {
     # A change to a .gyp, .gypi, or to GYP itself should run the generator.
     'name': 'gyp',
     'pattern': '.',