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/samplecode/SampleCamera.cpp b/samplecode/SampleCamera.cpp
index 1f3b8c3..991013a 100644
--- a/samplecode/SampleCamera.cpp
+++ b/samplecode/SampleCamera.cpp
@@ -57,7 +57,7 @@
 
 protected:
     // overrides from SkEventSink
-    virtual bool onQuery(SkEvent* evt) {
+    bool onQuery(SkEvent* evt) override {
         if (SampleCode::TitleQ(*evt)) {
             SampleCode::TitleR(evt, "Camera");
             return true;
@@ -65,7 +65,7 @@
         return this->INHERITED::onQuery(evt);
     }
 
-    virtual void onDrawContent(SkCanvas* canvas) {
+    void onDrawContent(SkCanvas* canvas) override {
         canvas->translate(this->width()/2, this->height()/2);
 
         Sk3DView    view;