[Extensions] Update extensions unittests to use manifest v2

A bunch of extension unittests generate extension objects at runtime.
Update these to generate extensions with manifest_version: 2. Where
trivial, update these to use ExtensionBuilder (if they weren't already),
which uses manifest v2 by default.

Bug: 816679
Bug: 756488

Change-Id: Iec80a5dd288452e70e5dbdd1902180611f6bbcca
Reviewed-on: https://chromium-review.googlesource.com/1007501
Commit-Queue: Devlin <[email protected]>
Reviewed-by: Karan Bhatia <[email protected]>
Cr-Commit-Position: refs/heads/master@{#550180}
diff --git a/extensions/browser/requirements_checker_unittest.cc b/extensions/browser/requirements_checker_unittest.cc
index 0beb5c6..eaa5234 100644
--- a/extensions/browser/requirements_checker_unittest.cc
+++ b/extensions/browser/requirements_checker_unittest.cc
@@ -56,6 +56,7 @@
   void CreateExtension() {
     manifest_dict_->SetString("name", "dummy name");
     manifest_dict_->SetString("version", "1");
+    manifest_dict_->SetInteger("manifest_version", 2);
 
     std::string error;
     extension_ =