Fix com.android.dex.DexIndexOverflowException when generating

system_webview_shell_page_cycler_apk.

No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 821914
Change-Id: I268530f8624710dd626a4578002a98b858e36008
Reviewed-on: https://chromium-review.googlesource.com/962829
Commit-Queue: Xi Han <hanxi@chromium.org>
Reviewed-by: agrieve <agrieve@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543147}

android_webview/tools/system_webview_shell/BUILD.gn

@@ -58,6 +58,7 @@ instrumentation_test_apk("system_webview_shell_page_cycler_apk") {
     "//third_party/android_support_test_runner:rules_java",
     "//third_party/android_support_test_runner:runner_java",
   ]
+  enable_multidex = true
 }
 
 instrumentation_test_apk("system_webview_shell_layout_test_apk") {