Bug 1813494 - Part 3: Add more cancellation checks and state assertions to module load requests r=smaug

We should check for previous cancellation on all path where a module load
request's state can be changed by an asynchronous action. Also add assertions
about the expected previous state where possible.

Differential Revision: https://phabricator.services.mozilla.com/D168233

js/loader/ScriptLoadRequest.h

@@ -209,8 +209,8 @@ class ScriptLoadRequest
   };
 
   bool IsFetching() const { return mState == State::Fetching; }
-
   bool IsCompiling() const { return mState == State::Compiling; }
+  bool IsLoadingImports() const { return mState == State::LoadingImports; }
 
   bool IsReadyToRun() const {
     return mState == State::Ready || mState == State::Canceled;