[headless] Use individual aura::WindowTreeHosts per WebContents.

headless/lib/headless_web_contents_browsertest.cc

Index: headless/lib/headless_web_contents_browsertest.cc
diff --git a/headless/lib/headless_web_contents_browsertest.cc b/headless/lib/headless_web_contents_browsertest.cc
index 67fee60f2086f5f0d404b30be5e08fb45c5130e1..50a493769fec9eb6851cfc02349d6b0715c8f2eb 100644
--- a/headless/lib/headless_web_contents_browsertest.cc
+++ b/headless/lib/headless_web_contents_browsertest.cc
@@ -86,19 +86,12 @@ IN_PROC_BROWSER_TEST_F(HeadlessWebContentsTest, Focus) {
           .Build();
   EXPECT_TRUE(WaitForLoad(web_contents2));
 
-  // TODO(irisu): Focus of two web contents should be independent of the other.
-  // Both web_contents and web_contents2 should be focused at this point.
-  // NOTE(dvallet): Mac headless implementation already does this.
+  // Focus of different WebContents is independent.
   EXPECT_TRUE(EvaluateScript(web_contents, "document.hasFocus()")
                   ->GetResult()
                   ->GetValue()
                   ->GetAsBoolean(&result));
-#if !defined(OS_MACOSX)
-  EXPECT_FALSE(result);
-#else
-  // Mac sets all contents as focused.
   EXPECT_TRUE(result);
-#endif
   EXPECT_TRUE(EvaluateScript(web_contents2, "document.hasFocus()")
                   ->GetResult()
                   ->GetValue()