Bug 1380227 - Avoid many UTF16toUTF8 and UTF8toUTF16 conversions in nsStringBundle. r=emk.

Most of the names passed to nsIStringBundle::{Get,Format}StringFromUTF8Name
have one of the two following forms:

- a 16-bit C string literal, which is then converted to an 8-bit string in
  order for the lookup to occur;

- an 8-bit C string literal converted to a 16-bit string, which is then
  converted back to an 8-bit string in order for the lookup to occur.

This patch introduces and uses alternative methods that can take an 8-bit C
string literal, which requires changing some signatures in other methods and
functions. It replaces all C++ uses of the old methods.

The patch also changes the existing {Get,Format}StringFromName() methods so
they take an AUTF8String argument for the name instead of a wstring, because
that's nicer for JS code.

Even though there is a method for C++ code and a different one for JS code,
|binaryname| is used so that the existing method names can be used for the
common case in both languages.

The change reduces the number of NS_ConvertUTF8toUTF16 and
NS_ConvertUTF16toUTF8 conversions while running Speedometer v2 from ~270,000 to
~160,000. (Most of these conversions involved the string
"deprecatedReferrerDirective" in nsCSPParser.cpp.)

dom/html/HTMLInputElement.cpp

@@ -7609,15 +7609,15 @@ HTMLInputElement::SetFilePickerFiltersFromAccept(nsIFilePicker* filePicker)
     // First, check for image/audio/video filters...
     if (token.EqualsLiteral("image/*")) {
       filterMask = nsIFilePicker::filterImages;
-      filterBundle->GetStringFromName(u"imageFilter",
+      filterBundle->GetStringFromName("imageFilter",
                                       getter_Copies(extensionListStr));
     } else if (token.EqualsLiteral("audio/*")) {
       filterMask = nsIFilePicker::filterAudio;
-      filterBundle->GetStringFromName(u"audioFilter",
+      filterBundle->GetStringFromName("audioFilter",
                                       getter_Copies(extensionListStr));
     } else if (token.EqualsLiteral("video/*")) {
       filterMask = nsIFilePicker::filterVideo;
-      filterBundle->GetStringFromName(u"videoFilter",
+      filterBundle->GetStringFromName("videoFilter",
                                       getter_Copies(extensionListStr));
     } else if (token.First() == '.') {
       if (token.Contains(';') || token.Contains('*')) {