Add DEPS entry for common repo

BUG=skia:
R=reed@google.com, rmistry@google.com

Author: borenet@google.com

Review URL: https://codereview.chromium.org/334633008
diff --git a/DEPS b/DEPS
index 9893ed1..f4bcada 100644
--- a/DEPS
+++ b/DEPS
@@ -3,6 +3,8 @@
 # Dependencies on outside packages.
 #
 deps = {
+  "common": "https://skia.googlesource.com/common.git@9bda9ca2f55b584189b83457d1cfea7805713f64",
+
   # DEPS using https://chromium.googlesource.com are pulled from chromium @ r205199
   # (see https://chromium.googlesource.com/chromium/chromium/+/c59bfa8ef877f45bfa859669053859857af1d279)
   "third_party/externals/angle2" : "https://chromium.googlesource.com/angle/angle.git",