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/gm/simpleaaclip.cpp b/gm/simpleaaclip.cpp
index c0aa515..1745eb1 100644
--- a/gm/simpleaaclip.cpp
+++ b/gm/simpleaaclip.cpp
@@ -126,7 +126,7 @@
         canvas->restore();
     }
 
-    virtual SkString onShortName() {
+    SkString onShortName() override {
         SkString str;
         str.printf("simpleaaclip_%s",
                     kRect_GeomType == fGeomType ? "rect" :
@@ -135,11 +135,11 @@
         return str;
     }
 
-    virtual SkISize onISize() {
+    SkISize onISize() override {
         return SkISize::Make(640, 480);
     }
 
-    virtual void onDraw(SkCanvas* canvas) {
+    void onDraw(SkCanvas* canvas) override {
 
         static const struct {
             SkColor         fColor;