[Android] Include NestedSystemMessageHandler in gn builds of content_shell_apk.

content/public/test/android/BUILD.gn

 import("//build/config/android/config.gni")
 import("//build/config/android/rules.gni")
 
 # GYP: //content/content_tests.gypi:test_support_content_jni_headers
 generate_jni("test_support_content_jni_headers") {
   jni_package = "content/public/test"
   sources = [
     "javatests/src/org/chromium/content/browser/test/NestedSystemMessageHandler.java",
   ]
 }
 
+android_library("layouttest_java_support") {
+  testonly = true
+  deps = [
+    "//base:base_java",
+  ]
+  java_files = [ "javatests/src/org/chromium/content/browser/test/NestedSystemMessageHandler.java" ]
+}
+
 # GYP: //content/content_tests.gypi:content_java_test_support
 android_library("content_java_test_support") {
+  testonly = true
   deps = [
+    ":layouttest_java_support",
     "//base:base_java",
     "//base:base_java_test_support",
     "//content/public/android:content_java",
   ]
   java_files = [
     "javatests/src/org/chromium/content/browser/test/NativeLibraryTestBase.java",
-    "javatests/src/org/chromium/content/browser/test/NestedSystemMessageHandler.java",
     "javatests/src/org/chromium/content/browser/test/util/ApplicationUtils.java",
     "javatests/src/org/chromium/content/browser/test/util/CallbackHelper.java",
     "javatests/src/org/chromium/content/browser/test/util/Criteria.java",
     "javatests/src/org/chromium/content/browser/test/util/CriteriaHelper.java",
     "javatests/src/org/chromium/content/browser/test/util/DOMUtils.java",
     "javatests/src/org/chromium/content/browser/test/util/EqualityCriteria.java",
     "javatests/src/org/chromium/content/browser/test/util/HistoryUtils.java",
     "javatests/src/org/chromium/content/browser/test/util/JavaScriptUtils.java",
     "javatests/src/org/chromium/content/browser/test/util/KeyUtils.java",
     "javatests/src/org/chromium/content/browser/test/util/MockLocationProvider.java",
     "javatests/src/org/chromium/content/browser/test/util/MockOrientationObserver.java",
     "javatests/src/org/chromium/content/browser/test/util/OrientationChangeObserverCriteria.java",
     "javatests/src/org/chromium/content/browser/test/util/RenderProcessLimit.java",
     "javatests/src/org/chromium/content/browser/test/util/TestCallbackHelperContainer.java",
     "javatests/src/org/chromium/content/browser/test/util/TestContentViewClient.java",
     "javatests/src/org/chromium/content/browser/test/util/TestInputMethodManagerWrapper.java",
     "javatests/src/org/chromium/content/browser/test/util/TestTouchUtils.java",
     "javatests/src/org/chromium/content/browser/test/util/TestWebContentsObserver.java",
     "javatests/src/org/chromium/content/browser/test/util/TouchCommon.java",
     "javatests/src/org/chromium/content/browser/test/util/UiUtils.java",
   ]
 }