Bug 1508302 - Make IMContextWrapper::GetIMEName() refer XMODIFIERS env when gtk_im_multicontext_get_context_id() returns "wayland" r=m_kato

On Wayland, `gtk_im_multicontext_get_context_id()` returns
`"wayland"`.  However, we need to know actual IM which works
behind Wayland.  Fortunately, `XMODIFIERS` env includes IME
name like "xim" case.  Therefore, we can refer it instead.

Differential Revision: https://phabricator.services.mozilla.com/D23469

--HG--
extra : moz-landing-system : lando

widget/gtk/IMContextWrapper.cpp

@@ -406,7 +406,8 @@ nsDependentCSubstring IMContextWrapper::GetIMName() const {
   // If the context is XIM, actual engine must be specified with
   // |XMODIFIERS=@im=foo|.
   const char* xmodifiersChar = PR_GetEnv("XMODIFIERS");
-  if (!im.EqualsLiteral("xim") || !xmodifiersChar) {
+  if (!xmodifiersChar ||
+      (!im.EqualsLiteral("xim") && !im.EqualsLiteral("wayland"))) {
     return im;
   }