[Android] Enable multidex for release builds of chrome_public_test_apk.

chrome/android/BUILD.gn

Index: chrome/android/BUILD.gn
diff --git a/chrome/android/BUILD.gn b/chrome/android/BUILD.gn
index c2b244d3f7081ebd8c6a97f955fa940bb71fc89a..1dcb3c61770635296d5d786060c6dc9db4bcf1d3 100644
--- a/chrome/android/BUILD.gn
+++ b/chrome/android/BUILD.gn
@@ -882,6 +882,11 @@ instrumentation_test_apk("chrome_public_test_apk") {
     "//net/android:net_test_support_apk",
   ]
   proguard_enabled = !is_java_debug
+
+  # The test APK contains code from both the APK under test and the
+  # test APK when proguard is enabled. That causes this APK to exceed
+  # the dex limit.
+  enable_multidex = proguard_enabled
 }
 
 if (enable_vr) {