[Android] Add enable_chrome_android_internal GN flag

This flag will allow devs having access to the internal Chrome for
Android repo to choose whether to use it in their builds.

BUG=702323

Review-Url: https://codereview.chromium.org/2753273002
Cr-Commit-Position: refs/heads/master@{#457749}
diff --git a/BUILD.gn b/BUILD.gn
index 8f84ca3..4ecf89c 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -322,7 +322,7 @@
       ]
     }
 
-    if (has_chrome_android_internal) {
+    if (enable_chrome_android_internal) {
       deps += [ "//clank" ]
     }
   }