Speculative fix for potential NULL pointer in PrintWarningToConsole

Couldn't reproduce this clusterfuzz bug locally. But make a speculative
fix attempt.

Bug: 1008300
Change-Id: Iab02f2b3997883ae05cccd1e1ea916a206952b60
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1838547Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Shrek Shao <shrekshao@google.com>
Cr-Commit-Position: refs/heads/master@{#703036}

third_party/blink/renderer/modules/webgl/webgl_rendering_context_base.cc

@@ -7516,9 +7516,12 @@ void WebGLRenderingContextBase::PrintGLErrorToConsole(const String& message) {
 }
 
 void WebGLRenderingContextBase::PrintWarningToConsole(const String& message) {
-  Host()->GetTopExecutionContext()->AddConsoleMessage(
-      ConsoleMessage::Create(mojom::ConsoleMessageSource::kRendering,
-                             mojom::ConsoleMessageLevel::kWarning, message));
+  blink::ExecutionContext* context = Host()->GetTopExecutionContext();
+  if (context) {
+    context->AddConsoleMessage(
+        ConsoleMessage::Create(mojom::ConsoleMessageSource::kRendering,
+                               mojom::ConsoleMessageLevel::kWarning, message));
+  }
 }
 
 bool WebGLRenderingContextBase::ValidateFramebufferFuncParameters(