Add support for field trial config for Isolate Extensions.

chrome/common/extensions/extension_process_policy.cc

Index: chrome/common/extensions/extension_process_policy.cc
diff --git a/chrome/common/extensions/extension_process_policy.cc b/chrome/common/extensions/extension_process_policy.cc
index 914427763bfd510328b9217a8b9d0df3e4cdd7e8..0bb15915d62ec6a0ff7215d17537e3d813502aa4 100644
--- a/chrome/common/extensions/extension_process_policy.cc
+++ b/chrome/common/extensions/extension_process_policy.cc
@@ -68,7 +68,11 @@ bool CrossesExtensionProcessBoundary(
 }
 
 bool IsIsolateExtensionsEnabled() {
-  return true;
+  const std::string group_name =
+      base::FieldTrialList::FindFullName("SiteIsolationExtensions");
+  bool is_control_group = base::StartsWith(
+      group_name, "Control", base::CompareCase::INSENSITIVE_ASCII);

[ncarter (slow), 2016/10/13 20:39:45]

Is Control already at zero percent for M55+?

What behavior do we get if a browser starts up with no internet connection, and
hence no available finch config?

[nasko, 2016/10/13 22:22:40]

Yes.
I think it falls back to previously cached response from the server.

+  return !is_control_group;
 }
 
 }  // namespace extensions