Make --site-per-process override --process-per-tab in RFHM::ShouldTransitionCrossSite.

This is necessary so that layout tests can run with --site-per-process.  Layout tests run with --dump-render-tree, which also forces --process-per-tab, and the latter was conflicting with --site-per-process.

BUG=417518

Review URL: https://codereview.chromium.org/891063002

Cr-Commit-Position: refs/heads/master@{#314985}

content/browser/frame_host/render_frame_host_manager.cc

@@ -844,6 +844,12 @@ bool RenderFrameHostManager::ResetProxiesInSiteInstance(int32 site_instance_id,
 }
 
 bool RenderFrameHostManager::ShouldTransitionCrossSite() {
+  // True for --site-per-process, which overrides both kSingleProcess and
+  // kProcessPerTab.
+  if (base::CommandLine::ForCurrentProcess()->HasSwitch(
+          switches::kSitePerProcess))
+    return true;
+
   // False in the single-process mode, as it makes RVHs to accumulate
   // in swapped_out_hosts_.
   // True if we are using process-per-site-instance (default) or