Bug 1397426 - Stop disabling tab warming in browser_bug343515.js. r=billm

Tab warming was originally disabled in this test because it was accidentally
setting the DocShell to be active at an unexpected time.

Because tab warming no longer activates DocShells, we can revert this change,
though we also have to make it deal with the fact that switching tabs no longer
makes docShells become activate immediately.

MozReview-Commit-ID: K0fiA5AVfEm

--HG--
extra : rebase_source : 5de92eb125e1001dd358a5deaa76f626802e3928

docshell/test/navigation/browser_bug343515.js

@@ -38,23 +38,17 @@ function test() {
   // Lots of callbacks going on here
   waitForExplicitFinish();
 
-  // Begin the test. First, we disable tab warming because it's
-  // possible for the mouse to be over one of the tabs during
-  // this test, warm it up, and cause this test to fail intermittently
-  // in automation.
-  SpecialPowers.pushPrefEnv({
-    set: [["browser.tabs.remote.warmup.enabled", false]],
-  }).then(step1);
+  // Begin the test
+  step1();
 }
 
-function step1() {
-
+async function step1() {
   // Get a handle on the initial tab
   ctx.tab0 = gBrowser.selectedTab;
   ctx.tab0Browser = gBrowser.getBrowserForTab(ctx.tab0);
 
-  // Our current tab should be active
-  ok(ctx.tab0Browser.docShellIsActive, "Tab 0 should be active at test start");
+  await BrowserTestUtils.waitForCondition(() => ctx.tab0Browser.docShellIsActive,
+    "Timed out waiting for initial tab to be active.");
 
   // Open a New Tab
   ctx.tab1 = BrowserTestUtils.addTab(gBrowser, testPath + "bug343515_pg1.html");