Disable Goma for Linux32 (ARM) bots.

This config is having issues to build reliably on Goma.

BUG=chromium:726706
TBR=mbonadei@webrtc.org

Review-Url: https://codereview.webrtc.org/2911893002 .
Cr-Commit-Position: refs/heads/master@{#18295}

tools_webrtc/mb/mb_config.pyl

@@ -201,8 +201,9 @@
     'gcc_release_bot_x64': [
       'gn', 'gcc', 'release_bot', 'x64'
     ],
+    # TOOD(kjellander): Restore Goma for this when crbug.com/726706 is fixed.
     'debug_bot_arm': [
-      'gn', 'openh264_debug_bot', 'arm'
+      'gn', 'openh264', 'debug', 'arm'
     ],
     'release_bot_arm': [
       'gn', 'openh264_release_bot', 'arm'