Bug 1569859 - [devtools] Always return sync content when using SourcesManager.urlContents with partial=true. r=nchevobbe

With popup debugging (next patches), we trigger a race condition in this code
where `SourcesManager.urlContents` is called *after* `devtools-html-content` is fired.
i.e. after the HTML document is parsed.
This lead to return an async promise instead of an immediate value.
This confuses `SourceActor._getStartLineColumnDisplacement` which no longer apply breakpoints right away.
We miss early breakpoint support for popups.

This isn't easy to reproduce beyond popup debugging,
in next changeset, browser_dbg-breakpoints-popup.js's testPausedByBreakpoint covers this.

Differential Revision: https://phabricator.services.mozilla.com/D135144

devtools/client/debugger/test/mochitest/browser_dbg-inline-script-offset.js

@@ -8,6 +8,7 @@
 
 add_task(async function() {
   const dbg = await initDebugger("doc-inline-script-offset.html");
+  await selectSource(dbg, "doc-inline-script-offset.html");
   await addBreakpoint(dbg, "doc-inline-script-offset.html", 6, 66);
   await reload(dbg);
   await waitForPaused(dbg);

devtools/server/actors/utils/sources-manager.js

@@ -408,7 +408,12 @@ class SourcesManager extends EventEmitter {
         contentType: data.contentType,
       };
     }
-
+    if (partial) {
+      return {
+        content: "",
+        contentType: "",
+      };
+    }
     return this._fetchURLContents(url, partial, canUseCache);
   }