Fix crash in 2D canvas caused by lost context

BUG=854068

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ie19b1595538a6dad8d8c095233fe16d9383cfd00
Reviewed-on: https://chromium-review.googlesource.com/1108347Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Commit-Queue: Justin Novosad <junov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568959}

third_party/blink/renderer/platform/graphics/canvas_2d_layer_bridge.cc

@@ -525,7 +525,10 @@ void Canvas2DLayerBridge::FlushRecording() {
 
     // Rastering the recording would have locked images, since we've flushed
     // all recorded ops, we should relase all locked images as well.
-    GetOrCreateResourceProvider()->ReleaseLockedImages();
+    // A new null check on the resource provider is necessary just in case
+    // the playback crashed the context.
+    if (GetOrCreateResourceProvider())
+      GetOrCreateResourceProvider()->ReleaseLockedImages();
 
     if (is_deferral_enabled_)
       StartRecording();