[Chromecast] Export JNI headers.

This change ensures that JNI symbols are exported for
Cast android libs.

Bug: internal b/64113798
Test: Verify Cast no longer crashes.
Change-Id: Ic0dea004d60c7e5d161281921a70454f6a69bbb1
Reviewed-on: https://chromium-review.googlesource.com/590076
Reviewed-by: Alok Priyadarshi <alokp@chromium.org>
Commit-Queue: Doug Horn <doughorn@google.com>
Cr-Commit-Position: refs/heads/master@{#490235}

diff --git a/chromecast/chromecast.gni b/chromecast/chromecast.gni
index bd4cdfff..806630a 100644
--- a/chromecast/chromecast.gni
+++ b/chromecast/chromecast.gni

@@ -103,6 +103,9 @@
 
 set_defaults("cast_shared_library") {
   configs = default_shared_library_configs
+  if (is_android) {
+    configs -= [ "//build/config/android:hide_all_but_jni_onload" ]
+  }
 }
 
 set_defaults("cast_loadable_module") {