Skip SiteInstanceTest.StartIsolatingSite if dynamic isolated origins are disabled.

Running content_unittests with --disable-site-isolation-trials caused
this test to fail because the flag disbles dynamic isolated origins
support. This change simply adds logic to skip the test if the feature
it is testing is disabled.

Bug: 1015882
Change-Id: I9a051251bce0d164943d72b2cb5c67887035f24b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2292794
Commit-Queue: Aaron Colwell <acolwell@chromium.org>
Auto-Submit: Aaron Colwell <acolwell@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#787480}

content/browser/site_instance_impl_unittest.cc

@@ -28,6 +28,7 @@
 #include "content/browser/webui/content_web_ui_controller_factory.h"
 #include "content/browser/webui/web_ui_controller_factory_registry.h"
 #include "content/public/browser/browser_or_resource_context.h"
+#include "content/public/browser/site_isolation_policy.h"
 #include "content/public/common/bindings_policy.h"
 #include "content/public/common/content_client.h"
 #include "content/public/common/content_constants.h"
@@ -1349,6 +1350,10 @@ TEST_F(SiteInstanceTest, IsOriginLockASite) {
 }
 
 TEST_F(SiteInstanceTest, StartIsolatingSite) {
+  // Skip this test case if dynamic isolated origins are not enabled.
+  if (!SiteIsolationPolicy::AreDynamicIsolatedOriginsEnabled())
+    return;
+
   IsolationContext isolation_context(context());
   auto* policy = ChildProcessSecurityPolicyImpl::GetInstance();