AutoplayPolicy::ShouldAutoplay should be tolerant of a null ExecutionContext

Bug: 1112031
Test: fast/media/readystate-update-in-detached-context.html
Change-Id: Id3c865021b1331d83004b371bdf5b9b23d879e84
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2335484Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Nate Chapin <japhet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#794572}

third_party/blink/renderer/core/html/media/autoplay_policy.cc

@@ -381,7 +381,8 @@ void AutoplayPolicy::MaybeSetAutoplayInitiated() {
 }
 
 bool AutoplayPolicy::ShouldAutoplay() {
-  if (element_->GetExecutionContext()->IsSandboxed(
+  if (!element_->GetExecutionContext() ||
+      element_->GetExecutionContext()->IsSandboxed(
           network::mojom::blink::WebSandboxFlags::kAutomaticFeatures)) {
     return false;
   }

third_party/blink/web_tests/fast/media/readystate-update-in-detached-context-expected.txt

+PASS if no crash. 

third_party/blink/web_tests/fast/media/readystate-update-in-detached-context.html

+PASS if no crash.
+<noscript>
+<iframe></iframe>
+</noscript>
+<object id='o'>
+<video id='v1'></video>
+</object>
+<object data='http:///'>
+<video id='v2' src='data:video/webm;base64,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'>
+</video>
+<script>
+if (window.testRunner)
+  testRunner.dumpAsText();
+
+document.addEventListener("DOMContentLoaded", () => {
+  o.width = 0;
+  window.frames[0].document.body.appendChild(v2);
+  var v1 = document.getElementById('v1');
+  o.removeChild(v1);
+});
+</script>