To further the investigation of issue 561873, as discussed with Ken Rockot in https://groups.google.com/a/chromium.org/forum/#!topic/chromium-mojo/DtD1-jaRFpo and in a private thread, we'll try the following: Assert early if the DWriteFontProxy::FindFamily call fails. 

According to Ken, the only two reasons why this call would return false are:

* The message was malformed and rejected by the receiving end because it failed validation. [...]
* The remote end of the interface is closed for whatever reason. [...]

We insert an assertion in https://cs.chromium.org/chromium/src/content/child/dwrite_font_proxy/dwrite_font_proxy_win.cc?q=dwrite_font_proxy_win.cc&sq=package:chromium&dr&l=107 and assert early if this fails in order to gather more information about in which clients this fails.