Make DETECT_IMPORTANT_CONTENT the default Plugins content setting based

chrome/test/BUILD.gn

Index: chrome/test/BUILD.gn
diff --git a/chrome/test/BUILD.gn b/chrome/test/BUILD.gn
index a3361ddd870347da44716e65cd14e2d89132062a..602bf3b8c22f10fed53cad3e2c0dc8cd69957184 100644
--- a/chrome/test/BUILD.gn
+++ b/chrome/test/BUILD.gn
@@ -684,6 +684,9 @@ if (!is_android) {
       deps += [ "//third_party/cld_2:cld2_platform_impl" ]
     }
 
+    if (enable_plugins) {
+      sources += chrome_tests_gypi_values.chrome_browser_tests_plugins_sources
+    }
     if (!enable_one_click_signin) {
       sources -= [ "../browser/ui/sync/one_click_signin_bubble_links_delegate_browsertest.cc" ]
     }