[build] Enable shared libraries for arm debug

BUG=chromium:648864,chromium:648610
NOTRY=true
TBR=jgruber@chromium.org

Review-Url: https://codereview.chromium.org/2371713002
Cr-Commit-Position: refs/heads/master@{#39712}

infra/mb/mb_config.pyl

@@ -259,7 +259,9 @@
 
     # GN debug configs for arm.
     'gn_debug_arm': [
-      'gn', 'debug_bot', 'arm', 'crosscompile', 'hard_float', 'swarming'],
+      'gn', 'debug', 'arm',
+      'shared', 'goma', 'v8_enable_slow_dchecks', 'v8_optimized_debug',
+      'crosscompile', 'hard_float', 'swarming'],
 
     # GN release configs for arm.
     'gn_release_arm': [