Bug 1156742 Part 9: Add a new nsIDeviceContextSpec for proxied printing. r=roc

This also changes aPrintToFileName parameter for BeginDocument to an nsAString& from char16_t*.
Having a char16_t* caused a pain with VS2105 where wchar_t != char16_t (as on VS2103), after it had been sent over IPDL.
This could have been worked around with casting, but this seemed like the tidier solution.

widget/android/nsDeviceContextAndroid.cpp

@@ -52,7 +52,7 @@ nsDeviceContextSpecAndroid::Init(nsIWidget* aWidget,
 
 NS_IMETHODIMP
 nsDeviceContextSpecAndroid::BeginDocument(const nsAString& aTitle,
-                                          char16_t* aPrintToFileName,
+                                          const nsAString& aPrintToFileName,
                                           int32_t aStartPage,
                                           int32_t aEndPage)
 {