Presubmit: Skip third_party for fwd decl warning

Skip presubmit warning when the introduced useless forward
declaration is made in third_party (with the exception of
blink). Take the opportunity to fix one test and style.

BUG=662195
TEST=PRESUBMIT_test.py ForwardDeclarationTest

Review-Url: https://codereview.chromium.org/2568473002
Cr-Commit-Position: refs/heads/master@{#437833}

diff --git a/PRESUBMIT_test_mocks.py b/PRESUBMIT_test_mocks.py
index 37b0159..c269deec 100644
--- a/PRESUBMIT_test_mocks.py
+++ b/PRESUBMIT_test_mocks.py

@@ -120,7 +120,7 @@
     return self._local_path
 
   def GenerateScmDiff(self):
-    return self._scm_diff;
+    return self._scm_diff
 
   def rfind(self, p):
     """os.path.basename is called on MockFile so we need an rfind method."""