Make sure metrics are recorded in HTMLCanvasElement dispose

The call to DiscardResourceProvider is causing a lot of false negatives.
https://uma.googleplex.com/p/chrome/histograms?sid=03450a933557bea663d8de9368d8d279

"Blink.Canvas.IsComposited" is redundant with "IsAccelerated", so I'm
just getting rid of it.

Bug: 1165879a
Change-Id: Ib3d1ce31d5225215155378dfb1eecaa95e4c7b48
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2625289Reviewed-by: Juanmi Huertas <juanmihd@chromium.org>
Reviewed-by: Aaron Krajeski <aaronhk@chromium.org>
Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Commit-Queue: Aaron Krajeski <aaronhk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843525}

third_party/blink/renderer/core/html/canvas/html_canvas_element.cc

@@ -142,9 +142,10 @@ HTMLCanvasElement::~HTMLCanvasElement() {
 }
 
 void HTMLCanvasElement::Dispose() {
-  if (OffscreenCanvasFrame()) {
-    ReleaseOffscreenCanvasFrame();
-  }
+  // We need to record metrics before we dispose of anything
+  if (context_)
+    UMA_HISTOGRAM_BOOLEAN("Blink.Canvas.HasRendered", bool(ResourceProvider()));
+
   // It's possible that the placeholder frame has been disposed but its ID still
   // exists. Make sure that it gets unregistered here
   UnregisterPlaceholderCanvas();
@@ -154,12 +155,8 @@ void HTMLCanvasElement::Dispose() {
   DiscardResourceProvider();
 
   if (context_) {
-    UMA_HISTOGRAM_BOOLEAN("Blink.Canvas.HasRendered", bool(ResourceProvider()));
-    if (context_->Host()) {
-      UMA_HISTOGRAM_BOOLEAN("Blink.Canvas.IsComposited",
-                            context_->IsComposited());
+    if (context_->Host())
       context_->DetachHost();
-    }
     context_ = nullptr;
   }