rebaseline_server: allow users to generate SKP diff patches on a shared instance

This will allow developers to maintain SKP baselines without ever running their
own rebaseline_server instance!

For now, the developer must manually apply the resulting patchset to his local
Skia checkout to actually modify expectations; in the near future, we hope to
make the UI upload the patchset to Rietveld on the user's behalf.

BUG=skia:1918
NOTRY=true
R=stephana@google.com, rmistry@google.com

Author: epoger@google.com

Review URL: https://codereview.chromium.org/487853004
diff --git a/DEPS b/DEPS
index a3d7a3c..9fb9078 100644
--- a/DEPS
+++ b/DEPS
@@ -3,7 +3,7 @@
 # Dependencies on outside packages.
 #
 deps = {
-  "common": "https://skia.googlesource.com/common.git@a7b0283c39dfdf5553859eb9790c970bb52cda47",
+  "common": "https://skia.googlesource.com/common.git@5dd2918f96b277ebe04eb66469ec3d5c7ba6952a",
 
   "third_party/externals/angle2" : "https://chromium.googlesource.com/angle/angle.git@23a8a433529d9db23882c702a29d5e594841563d",
   "third_party/externals/freetype" : "https://skia.googlesource.com/third_party/freetype2.git@VER-2-5-0-1",