nsFormControlHelper.h not found

I am trying to fix Bug ID: 209097 [Default mailto form subject
should be localizable]
Ref: http://bugzilla.mozilla.org/show_bug.cgi?id=209097

This is my first bug, so would appreciate any help. As suggested
by [email protected], I added the default subject as a localizable
string in the file: layout/html/forms/src/HtmlForm.properties
i.e. added the following line:
DefaultSubject=Formular wurde über Mozilla versendet

To retrieve this string at:
http://lxr.mozilla.org/mozilla/source/content/html/content/src/nsFormSubmission.cpp#450
where it is used, I tried to use
nsFormControlHelper::GetLocalizedString
the way it is used at:
http://lxr.mozilla.org/mozilla/source/layout/html/forms/src/nsFileControlFrame.cpp#313
For this, I did '#include "nsFormControlHelper.h"' in
nsFormSubmission.cpp
(Without that, nsFormControlHelper was an unknown symbol.)

However, the .h file was not found at compile time. It is at:
http://lxr.mozilla.org/mozilla/source/layout/html/forms/src/nsFormControlHelper.h
(which is in the same dir as nsFileControlFrame.cpp where it is
successfully
used, so it might have worked there for this reason.)
What puzzles me is that there is another file in a different dir where
it
is also included successfully, which is:
http://lxr.mozilla.org/mozilla/source/layout/html/style/src/nsCSSFrameConstructor.cpp#104
http://lxr.mozilla.org/mozilla/source/layout/html/style/src/nsCSSFrameConstructor.cpp#10170

So, I guess, the question in short is: How do I get access to
nsFormControlHelper in content/html/content/src/nsFormSubmission.cpp ?

Thanks for reading through the somewhat involved description, and
for any help you can provide.
- Ajoy