[filesapp] Change startInitBackgroundPage_ for files app SWA case

Initialize this.backgroundPage_ with new BackgroundWindowSWA() fake if
Files app is running in the SWA container.

window.BackgroundWindowSWA || null is used to pass Closure compile for
current Files app (there are no extern defines for the SWA fakes).

No change in Files app behavior.

Test: browser_tests --gtest_filter="*launchFilesAppSwa*"
Bug: 1113981
Change-Id: Ia079ea266583afd5a2ea67412758a77939327c8b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2542931Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Alex Danilo <adanilo@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828068}

ui/file_manager/file_manager/foreground/js/file_manager.js

@@ -847,12 +847,17 @@ class FileManager extends cr.EventTarget {
   async startInitBackgroundPage_() {
     metrics.startInterval('Load.InitBackgroundPage');
 
-    /** @type {!Window} */
-    const backgroundPage =
-        await new Promise(resolve => chrome.runtime.getBackgroundPage(resolve));
-    assert(backgroundPage);
-    this.backgroundPage_ =
-        /** @type {!BackgroundWindow} */ (backgroundPage);
+    /** @type {!BackgroundWindow} */
+    this.backgroundPage_ = await new Promise(resolve => {
+      if (window.isSWA) {
+        const backgroundWindowSWA = window.BackgroundWindowSWA || null;
+        resolve(new backgroundWindowSWA());
+      } else {
+        chrome.runtime.getBackgroundPage(resolve);
+      }
+    });
+
+    assert(this.backgroundPage_);
     this.fileBrowserBackground_ =
         /** @type {!FileBrowserBackgroundFull} */ (
             this.backgroundPage_.background);
@@ -868,6 +873,7 @@ class FileManager extends cr.EventTarget {
     this.mediaScanner_ = this.fileBrowserBackground_.mediaScanner;
     this.historyLoader_ = this.fileBrowserBackground_.historyLoader;
     this.crostini_ = this.fileBrowserBackground_.crostini;
+
     metrics.recordInterval('Load.InitBackgroundPage');
   }