Allow override of Gold diff instance

Stops hardcoding an appended '-public' onto the instance we use for
the Diff step in the Gold code, instead calling a method to get which
instance to use. This is because not all uses of Gold, e.g. ANGLE's,
can use the public instance (because it doesn't exist). These cases
can override the method to return the base/internal instance instead.

Bug: skia:10778
Change-Id: I94de33cac9a7f6935ae1507461c75be0fbc01170
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2436649
Auto-Submit: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811548}

build/skia_gold_common/skia_gold_session.py

@@ -345,10 +345,7 @@ class SkiaGoldSession(object):
           '--corpus',
           self._corpus,
           '--instance',
-          # TODO(skbug.com/10610): Decide whether to use the public or
-          # non-public instance once authentication is fixed for the non-public
-          # instance.
-          str(self._instance) + '-public',
+          self._GetDiffGoldInstance(),
           '--input',
           png_file,
           '--test',
@@ -470,6 +467,17 @@ class SkiaGoldSession(object):
   def _CreateDiffOutputDir(self):
     return tempfile.mkdtemp(dir=self._working_dir)
 
+  def _GetDiffGoldInstance(self):
+    """Gets the Skia Gold instance to use for the Diff step.
+
+    This can differ based on how a particular instance is set up, mainly
+    depending on whether it is set up for internal results or not.
+    """
+    # TODO(skbug.com/10610): Decide whether to use the public or
+    # non-public instance once authentication is fixed for the non-public
+    # instance.
+    return str(self._instance) + '-public'
+
   def _StoreDiffLinks(self, image_name, output_manager, output_dir):
     """Stores the local diff files as links.