re-guard against negative dimensions on no-draw canvas

We used to (incidentally) guard for this when we used bitmapdevice as our backnig.
Now that we have a (faster) nodrawdevice, we need to explicitly guard for it.

BUG=skia:

Change-Id: I9cbbf064cbfced78f0004a2e5aff60aa3ded6215
Reviewed-on: https://skia-review.googlesource.com/9530
Reviewed-by: Derek Sollenberger <djsollen@google.com>
Commit-Queue: Mike Reed <reed@google.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/include/core/SkSize.h b/include/core/SkSize.h
index 2efcfb59dc..153335da68 100644
--- a/include/core/SkSize.h
+++ b/include/core/SkSize.h
@@ -21,6 +21,10 @@ template <typename T> struct SkTSize {
         return s;
     }
 
+    static SkTSize MakeEmpty() {
+        return {0, 0};
+    }
+
     void set(T w, T h) {
         fWidth = w;
         fHeight = h;