[gn] Switch linux branch builder to gn

BUG=chromium:474921
NOTRY=true
TBR=jochen,vogelheim

Review-Url: https://codereview.chromium.org/2306573002
Cr-Commit-Position: refs/heads/master@{#39083}

infra/mb/mb_config.pyl

@@ -117,7 +117,7 @@
       'V8 Linux - beta branch - debug': 'gn_debug_x86',
       'V8 Linux - stable branch': 'gn_release_x86',
       'V8 Linux - stable branch - debug': 'gn_debug_x86',
-      'V8 Linux64 - beta branch': 'gyp_release_x64',
+      'V8 Linux64 - beta branch': 'gn_release_x64',
       'V8 Linux64 - beta branch - debug': 'gn_debug_x64',
       'V8 Linux64 - stable branch': 'gn_release_x64',
       'V8 Linux64 - stable branch - debug': 'gn_debug_x64',