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/ChecksumBench.cpp b/bench/ChecksumBench.cpp
index 9dd87c3..c731a8f 100644
--- a/bench/ChecksumBench.cpp
+++ b/bench/ChecksumBench.cpp
@@ -40,7 +40,7 @@
     }
 
 protected:
-    virtual const char* onGetName() {
+    const char* onGetName() override {
         switch (fType) {
             case kChecksum_ChecksumType: return "compute_checksum";
             case kMD5_ChecksumType: return "compute_md5";
@@ -51,7 +51,7 @@
         }
     }
 
-    virtual void onDraw(const int loops, SkCanvas*) {
+    void onDraw(const int loops, SkCanvas*) override {
         switch (fType) {
             case kChecksum_ChecksumType: {
                 for (int i = 0; i < loops; i++) {