[CCA] Add onAppWindowCreatedForTesting in background.

Use a callback instead of polling a boolean flag to notify the window
created event for test, because we might restart CCA when running a
test.

An alternative is to set the boolean flag to |false| in |onClosed|, but
that might hit a race condition if the app window is closed soon after
it's created, and before the |true| state is polled by test.

Bug: b:135905426
Test: Restart CCA 200 times in Tast.
Change-Id: Ia29e0221d072845b23e4598ca2d594330b3b1ac0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1672827Reviewed-by: Sheng-hao Tsao <shenghao@chromium.org>
Commit-Queue: Shik Chen <shik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672787}

chrome/browser/resources/chromeos/camera/src/js/background.js

@@ -40,9 +40,17 @@ cca.bg.TOPBAR_COLOR = '#000000';
  * won't connect to the main.html target before the window is created, otherwise
  * the window might disappear.
  * @type {boolean}
+ * @deprecated This flag would be removed after we migrate CCA Tast tests.
  */
 cca.bg.appWindowCreated = false;
 
+/**
+ * It's used in test to ensure that we won't connect to the main.html target
+ * before the window is created, otherwise the window might disappear.
+ * @type {?function(): undefined}
+ */
+cca.bg.onAppWindowCreatedForTesting = null;
+
 /**
  * Creates the window. Note, that only one window at once is supported.
  */
@@ -82,6 +90,9 @@ cca.bg.create = function() {
         chrome.storage.local.set({fullscreen: inAppWindow.isFullscreen()});
       });
       cca.bg.appWindowCreated = true;
+      if (cca.bg.onAppWindowCreatedForTesting !== null) {
+        cca.bg.onAppWindowCreatedForTesting();
+      }
     });
   });
 };