Create proguard for fake cronet

Fake cronet needs to keep the FakeCronetProvider class name so it can
be specified in the test resource files.

Bug: 669707
Change-Id: I3137bbb1eb78a7c56f4b0865b2a107b34cd43fda
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1763230Reviewed-by: Paul Jensen <pauljensen@chromium.org>
Commit-Queue: Benjamin Kirchman <kirchman@google.com>
Cr-Commit-Position: refs/heads/master@{#689438}

components/cronet/android/BUILD.gn

@@ -1348,6 +1348,7 @@ if (!is_component_build) {
       "//chrome/VERSION",
       "api_version.txt",
       "cronet_impl_common_proguard.cfg",
+      "cronet_impl_fake_proguard.cfg",
       "cronet_impl_platform_proguard.cfg",
     ]
     outputs = [

components/cronet/android/cronet_impl_fake_proguard.cfg

+# Proguard config for apps that depend on cronet_impl_fake_java.jar.
+
+# This constructor is called using the reflection from Cronet API (cronet_api.jar).
+-keep class org.chromium.net.test.FakeCronetProvider {
+    public <init>(android.content.Context);
+}