Simplify frame loads in OriginReplication and SandboxFlagsReplication tests.

Recent work on didStopLoading makes it possible for us to use
WaitForLoadStop to wait for all child frames to be loaded when
navigating a subframe.

BUG=

Review URL: https://codereview.chromium.org/1123233005

Cr-Commit-Position: refs/heads/master@{#330155}

content/browser/site_per_process_browsertest.cc

@@ -1505,16 +1505,10 @@ IN_PROC_BROWSER_TEST_F(SitePerProcessBrowserTest, OriginReplication) {
   TestNavigationObserver observer(shell()->web_contents());
 
   // Navigate the first subframe to a cross-site page with two subframes.
-  // NavigateFrameToURL can't be used here because it doesn't guarantee that
-  // FrameTreeNodes will have been created for child frames when it returns.
-  RenderFrameHostCreatedObserver frame_observer(shell()->web_contents(), 4);
   GURL foo_url(
       embedded_test_server()->GetURL("foo.com", "/frame_tree/1-1.html"));
-  NavigationController::LoadURLParams params(foo_url);
-  params.transition_type = ui::PAGE_TRANSITION_LINK;
-  params.frame_tree_node_id = root->child_at(0)->frame_tree_node_id();
-  root->child_at(0)->navigator()->GetController()->LoadURLWithParams(params);
-  frame_observer.Wait();
+  NavigateFrameToURL(root->child_at(0), foo_url);
+  EXPECT_TRUE(WaitForLoadStop(shell()->web_contents()));
 
   // We can't use a TestNavigationObserver to verify the URL here,
   // since the frame has children that may have clobbered it in the observer.
@@ -1591,13 +1585,11 @@ IN_PROC_BROWSER_TEST_F(SitePerProcessBrowserTest, SandboxFlagsReplication) {
   TestNavigationObserver observer(shell()->web_contents());
 
   // Navigate the second (sandboxed) subframe to a cross-site page with a
-  // subframe. Use RenderFrameHostCreatedObserver to guarantee that all
-  // FrameTreeNodes are created for child frames.
-  RenderFrameHostCreatedObserver frame_observer(shell()->web_contents(), 4);
+  // subframe.
   GURL foo_url(
       embedded_test_server()->GetURL("foo.com", "/frame_tree/1-1.html"));
   NavigateFrameToURL(root->child_at(1), foo_url);
-  frame_observer.Wait();
+  EXPECT_TRUE(WaitForLoadStop(shell()->web_contents()));
 
   // We can't use a TestNavigationObserver to verify the URL here,
   // since the frame has children that may have clobbered it in the observer.