Fix and enable lint checks.

This CL fixes and enables:
  * readability/namespace
  * readability/multiline_string
  * readability/multiline_comment
  * readability/inheritance
  * readability/function
  * readability/braces

R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/1747123002 .
diff --git a/samples/image_diff_png.cc b/samples/image_diff_png.cc
index 09913fb..93823aa 100644
--- a/samples/image_diff_png.cc
+++ b/samples/image_diff_png.cc
@@ -641,4 +641,4 @@
       std::vector<Comment>(), output);
 }
 
-}  // image_diff_png
+}  // namespace image_diff_png