Really enable DEBUG code on android unless full debug is specified

R=machenbach@chromium.rg
LOG=n
BUG=none
NOTRY=true

Review URL: https://codereview.chromium.org/786203003

Cr-Commit-Position: refs/heads/master@{#25721}

build/toolchain.gypi

@@ -1042,6 +1042,7 @@
                 # TODO(2304): pass DISABLE_DEBUG_ASSERT instead of hiding DEBUG.
                 'defines!': [
                   'DEBUG',
+                  'ENABLE_SLOW_DCHECKS',
                 ],
               }],
             ],