Fix up -Winconsistent-missing-override

  (and a couple presubmit fixes)

This allows us to turn back on -Werror for LLVM coverage builds,
and more generally supports building with Clang 3.7.

No public API changes.
TBR=reed@google.com

BUG=skia:

Review URL: https://codereview.chromium.org/1232463006
diff --git a/bench/RegionBench.cpp b/bench/RegionBench.cpp
index 84e8d1b..2188cb3 100644
--- a/bench/RegionBench.cpp
+++ b/bench/RegionBench.cpp
@@ -104,9 +104,9 @@
     }
 
 protected:
-    virtual const char* onGetName() { return fName.c_str(); }
+    const char* onGetName() override { return fName.c_str(); }
 
-    virtual void onDraw(const int loops, SkCanvas* canvas) {
+    void onDraw(const int loops, SkCanvas* canvas) override {
         Proc proc = fProc;
         for (int i = 0; i < loops; ++i) {
             proc(fA, fB);