Add Nexus 5X chromium.gpu bot to mb_config.pyl.

Forgot this the first time around.

BUG=786572
TBR=dpranke@chromium.org
NOTRY=true

Change-Id: I24cd1a3053e80a7e661f92f26d01011a6cf0bddb
Reviewed-on: https://chromium-review.googlesource.com/777853Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517695}

tools/mb/mb_config.pyl

@@ -271,6 +271,7 @@
       'GPU Linux Builder (dbg)': 'gpu_tests_debug_trybot',
       'GPU Win Builder': 'gpu_tests_release_trybot_x86_minimal_symbols',
       'GPU Win Builder (dbg)': 'gpu_tests_debug_trybot_x86_minimal_symbols',
+      'Android Release (Nexus 5X)': 'gpu_tests_android_release_trybot_arm64',
     },
 
     'chromium.gpu.fyi': {