Enable unused param checking for public includes.

This CL cleans up the existing violations and enables the
build time check to ensure that we don't regress.

The motiviation behind this change is to allow clients who include
our headers to be able to build with this warning enabled.

Review URL: https://codereview.chromium.org/726923002
diff --git a/include/core/SkShader.h b/include/core/SkShader.h
index 0fbc1b8..1515118 100644
--- a/include/core/SkShader.h
+++ b/include/core/SkShader.h
@@ -366,7 +366,7 @@
         const SkXfermode*   fMode;
     };
 
-    virtual bool asACompose(ComposeRec* rec) const { return false; }
+    virtual bool asACompose(ComposeRec*) const { return false; }
 
 
     /**