Bug 1798319 - Implement modulepreload in early hints r=manuel,smaug,necko-reviewers,kershaw

The aEarlyHintPreloaderId parameter for StartLoad/StartLoadInternal is changed
to be a member variable of ScriptLoadRequest instead so that an initiator type
of early hints can be set for module requests. Before, ModuleLoader would always
pass in a zero value for the id since ModuleLoaderBase has no concept of early
hints when it calls StartFetch.

As a prerequisite for early hints support, this commit also implements
modulepreload in link headers (Bug 1773056).

Differential Revision: https://phabricator.services.mozilla.com/D180020

js/loader/ScriptLoadRequest.cpp

@@ -88,7 +88,8 @@ ScriptLoadRequest::ScriptLoadRequest(ScriptKind aKind, nsIURI* aURI,
       mScriptBytecode(),
       mBytecodeOffset(0),
       mURI(aURI),
-      mLoadContext(aContext) {
+      mLoadContext(aContext),
+      mEarlyHintPreloaderId(0) {
   MOZ_ASSERT(mFetchOptions);
   if (mLoadContext) {
     mLoadContext->SetRequest(this);