[gcc 4.x] Initialize web_port_index to 0 to fix build error while using GCC 4.x toolchain.

media/midi/midi_manager_alsa.cc

Index: media/midi/midi_manager_alsa.cc
diff --git a/media/midi/midi_manager_alsa.cc b/media/midi/midi_manager_alsa.cc
index b6ce3f9dfb1bd324766d1ba4220866e547201ebc..3db6955c1ffcb60f510529bf0433399359da6ab2 100644
--- a/media/midi/midi_manager_alsa.cc
+++ b/media/midi/midi_manager_alsa.cc
@@ -446,7 +446,7 @@ MidiManagerAlsa::MidiPortState::MidiPortState()
 
 uint32 MidiManagerAlsa::MidiPortState::Insert(scoped_ptr<MidiPort> port) {
   // Add the web midi index.
-  uint32 web_port_index;
+  uint32 web_port_index = 0;
   switch (port->type()) {
     case MidiPort::Type::kInput:
       web_port_index = num_input_ports_++;