Don't disable the sysroot on GCC

BUG=chromium:575778
R=machenbach@chromium.org

Change-Id: Iac37a6a2f2ccf46467e4ecdbaefcc4e532491393
Reviewed-on: https://chromium-review.googlesource.com/576853
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46753}

infra/mb/mb_config.pyl

@@ -591,7 +591,7 @@
     },
 
     'gcc': {
-      'gn_args': 'is_clang=false use_sysroot=false',
+      'gn_args': 'is_clang=false',
       'gyp_defines': 'clang=0',
     },