teach rebaseline_server to generate diffs of rendered SKPs

Creates a new live-view.html page, served by the rebaseline_server, that will display diffs between two sets of rendered SKP images.

BUG=skia:1942
NOTRY=true
R=rmistry@google.com

Author: epoger@google.com

Review URL: https://codereview.chromium.org/424263005
diff --git a/DEPS b/DEPS
index 66ffc18..e6da509 100644
--- a/DEPS
+++ b/DEPS
@@ -3,7 +3,7 @@
 # Dependencies on outside packages.
 #
 deps = {
-  "common": "https://skia.googlesource.com/common.git@6d096df9431e61c163796c43adbf31a0c911a843",
+  "common": "https://skia.googlesource.com/common.git@f63e1cfff23615157e28942af5f5e8298351cb10",
 
   # DEPS using https://chromium.googlesource.com are pulled from chromium @ r205199
   # (see https://chromium.googlesource.com/chromium/chromium/+/c59bfa8ef877f45bfa859669053859857af1d279)