Fetch Body: fix deference of null ExecutionContext

The body attribute on the Body type accessed the ExecutionContext in
order to log a use counter, but didn't check if it was null. Add a null
check.

BUG=1034833

Change-Id: I424b1eae9140804b227d4c9a34bd75e494729722
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2000409
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#731909}

third_party/blink/renderer/core/fetch/body.cc

@@ -351,12 +351,13 @@ ScriptPromise Body::text(ScriptState* script_state,
 }
 
 ReadableStream* Body::body() {
-  auto* execution_context = GetExecutionContext();
-  if (execution_context->IsServiceWorkerGlobalScope()) {
-    execution_context->CountUse(WebFeature::kFetchBodyStreamInServiceWorker);
-  } else {
-    execution_context->CountUse(
-        WebFeature::kFetchBodyStreamOutsideServiceWorker);
+  if (auto* execution_context = GetExecutionContext()) {
+    if (execution_context->IsServiceWorkerGlobalScope()) {
+      execution_context->CountUse(WebFeature::kFetchBodyStreamInServiceWorker);
+    } else {
+      execution_context->CountUse(
+          WebFeature::kFetchBodyStreamOutsideServiceWorker);
+    }
   }
 
   if (!BodyBuffer())