When things get added to the log, like NS_WARNING output, it adds information about the process. For a content child process, it prints out that it is a child process: 17:51:19 INFO - [Child 1802] WARNING: ... Similarly in the parent process. However, for the geckomediaplugin, it just does: 17:56:48 INFO -  WARNING: ... The Gecko Media Plugin stuff is a new kind of process, so presumably there just needs to be a new case added somewhere. It would make interpreting the logs easier. I don't actually know where this code lives that produces that process output.
automationutils stuff uses geckomediaplugin as the name, but maybe just GMP here would be good, so the lines don't all get too long.
I think you just need a call like this in the GMPChild: http://hg.mozilla.org/mozilla-central/annotate/4d1793da0b96/dom/ipc/ContentChild.cpp#l524
Ah, great, I would not have guessed that.
Created attachment 8487428 [details] [diff] [review] Enable logging of GMP process name. I confirmed that this works, though it looks like some of the very early "WARNING: XPCOM objects created/destroyed from static ctor/dtor" spam doesn't get annotated. try run: https://tbpl.mozilla.org/?tree=Try&rev=929b76bb71f5