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/SampleFatBits.cpp b/samplecode/SampleFatBits.cpp
index 62d4a3d..9036fbe 100644
--- a/samplecode/SampleFatBits.cpp
+++ b/samplecode/SampleFatBits.cpp
@@ -446,7 +446,7 @@
         return this->INHERITED::onQuery(evt);
     }
 
-    virtual void onDrawContent(SkCanvas* canvas) {
+    void onDrawContent(SkCanvas* canvas) override {
         fFB.drawBG(canvas);
         if (fFB.getTriangle()) {
             fFB.drawTriangle(canvas, fPts);
@@ -473,8 +473,7 @@
         }
     }
 
-    virtual SkView::Click* onFindClickHandler(SkScalar x, SkScalar y,
-                                              unsigned modi) override {
+    SkView::Click* onFindClickHandler(SkScalar x, SkScalar y, unsigned modi) override {
         SkPoint pt = { x, y };
         int index = -1;
         int count = fFB.getTriangle() ? 3 : 2;