[build] Add build configs for experimental trybot

TBR=sergiyb@chromium.org
NOTRY=true

Bug: chromium:890222
Change-Id: I3c3f8902cc0775edfcdcc85bb0cbc3320bf6c1ef
Reviewed-on: https://chromium-review.googlesource.com/1251523
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56290}

infra/mb/mb_config.pyl

@@ -201,6 +201,7 @@
       'v8_linux_gcc_compile_rel': 'release_x86_gcc_minimal_symbols',
       'v8_linux_gcc_rel': 'release_x86_gcc_minimal_symbols',
       'v8_linux_shared_compile_rel': 'release_x86_shared_verify_heap',
+      'v8_linux64_compile_rel_xg': 'release_x64_test_features_trybot',
       'v8_linux64_dbg_ng': 'debug_x64_trybot',
       'v8_linux64_gcc_compile_dbg': 'debug_x64_gcc',
       'v8_linux64_header_includes_dbg': 'debug_x64_header_includes',