Make reset actually clear the backing buffer

This only should happen if the paint canvas exists so that it doesn't
wipe out the buffer in the situation when the context is lost.

Bug: 1131449, 1131137
Change-Id: I355de75bc92e41e8d2224bf4ecd4e9104b94a67b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2633345
Commit-Queue: Aaron Krajeski <aaronhk@chromium.org>
Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844597}

third_party/blink/renderer/modules/canvas/canvas2d/base_rendering_context_2d.cc

@@ -184,6 +184,9 @@ void BaseRenderingContext2D::reset() {
     DCHECK(c->getDeviceClipBounds(&clip_bounds));
     DCHECK(clip_bounds == c->imageInfo().bounds());
 #endif
+    // We only want to clear the backing buffer if the surface exists because
+    // this function is also used when the context is lost.
+    clearRect(0, 0, Width(), Height());
   }
   ValidateStateStack();
   origin_tainted_by_content_ = false;