Add an assert that we don't schedule a render during render

Add an assert that we don't schedule a render during render

In production I added an early return.  I don't bother to log
in production since no one is looking at the log anyway.

Unfortunately this currently only fires when using a debug build
we should fix our Release build to have a checked-mode option
or consider hacks like having all localhost urls enable checked
mode, etc.

R=ianh@google.com, rafaelw@chromium.org
BUG=

Committed: https://chromium.googlesource.com/external/mojo/+/356d3b720e51a626acf75626ae738648a2b171b6

[Hixie, 2015-03-06 18:42:35]

ianh@google.com changed reviewers:
+ ianh@google.com

[Hixie, 2015-03-06 18:42:35]

lgtm

[eseidel, 2015-03-06 18:42:52]

Committed patchset #2 (id:20001) manually as
356d3b720e51a626acf75626ae738648a2b171b6 (presubmit successful).