nsPrintfCString.h defines only one class: nsPrintfCString. Therefore, when a file include nsPrintfCString.h but uses nsPrintfCString nowhere, it's unneeded.
There is currently the case in many files: https://mxr.mozilla.org/mozilla-central/search?string=nsPrintfCString
Created attachment 543926 [details] [diff] [review]
Created attachment 544163 [details] [diff] [review]
same patch with a commit message
Btw, a commit message needs to include the bug number and the reviewer. Something like
Bug 669299 - Remove unneeded nsPrintfCString.h includes; r=bsmedberg