Enable undo-commit iff IBUS_CAP_SURROUNDING_TEXT is set.

Ibus doesn't have any appropriate capability flag to indicate
when we can reliably use |ibus_engine_delete_surrounding_text|.

With this CL, undo-commit is enabled more conservatively.  It now
requires the client to set IBUS_CAP_SURROUNDING_TEXT bit.

Closes #296.

BUG=#296
TEST=compile

src/mozc_version_template.txt

@@ -1,6 +1,6 @@
 MAJOR=2
 MINOR=17
-BUILD=2107
+BUILD=2108
 REVISION=102
 # NACL_DICTIONARY_VERSION is the target version of the system dictionary to be
 # downloaded by NaCl Mozc.

src/unix/ibus/mozc_engine.cc

@@ -716,14 +716,12 @@ bool MozcEngine::ExecuteCallback(IBusEngine *engine,
 
   switch (callback_command.type()) {
     case commands::SessionCommand::UNDO:
-      // As far as I've tested on Ubuntu 11.10, most of applications which
-      // accept 'ibus_engine_delete_surrounding_text' doe not set
-      // IBUS_CAP_SURROUNDING_TEXT bit.
-      // So we should carefully uncomment the following code.
-      // -----
-      // if (!(engine->client_capabilities & IBUS_CAP_SURROUNDING_TEXT)) {
-      //   return false;
-      // }
+      // Having |IBUS_CAP_SURROUNDING_TEXT| does not necessarily mean that the
+      // client supports |ibus_engine_delete_surrounding_text()|, but there is
+      // no other good criteria.
+      if (!(engine->client_capabilities & IBUS_CAP_SURROUNDING_TEXT)) {
+        return false;
+      }
       break;
     case commands::SessionCommand::CONVERT_REVERSE: {
       if (!GetSurroundingText(engine,