Draw native GTK window nativation buttons on Linux

This CL is the initial implementation of GTK-rendered close, minimize,
and maximize buttons.  A demo can be seen at [1].  The implementation
is not ready to be released yet (still need to handle different scale
factors and button drawing for "backdrop" windows), so the feature is
guarded by a flag for now.

[1] https://bugs.chromium.org/p/chromium/issues/detail?id=753067#c3

BUG=753067
[email protected],[email protected]

Change-Id: Iadb5c96052f100097a10a5161aa3d9d44d89c901
Reviewed-on: https://chromium-review.googlesource.com/604288
Commit-Queue: Thomas Anderson <[email protected]>
Reviewed-by: Scott Violet <[email protected]>
Reviewed-by: Elliot Glaysher <[email protected]>
Cr-Commit-Position: refs/heads/master@{#496031}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index df7f39b..e3c6f74 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -869,6 +869,9 @@
 extern const char kWifiCredentialSyncName[];
 extern const char kWifiCredentialSyncDescription[];
 
+extern const char kWindowNavNativeFlagName[];
+extern const char kWindowNavNativeFlagDescription[];
+
 extern const char kZeroCopyName[];
 extern const char kZeroCopyDescription[];