linux/gyp: Find missing symbols at build time, not runtime.

content/content_browser.gypi

Index: content/content_browser.gypi
diff --git a/content/content_browser.gypi b/content/content_browser.gypi
index f0b7d063e9f0eaf494f4a61ce3769f6aebfc7b5c..f76a32fbc9c93c5fd6ae67a7edc9b91abde70d4b 100644
--- a/content/content_browser.gypi
+++ b/content/content_browser.gypi
@@ -1811,6 +1811,8 @@
     ['use_x11==1', {
       'dependencies': [
         '../build/linux/system.gyp:x11',
+        '../ui/events/platform/x11/x11_events_platform.gyp:x11_events_platform',
+        '../ui/gfx/x/gfx_x11.gyp:gfx_x11',
       ],
     }],
     ['use_pango==1', {