Add suppressions for memcheck errors.

tools/valgrind-webrtc/memcheck/suppressions.txt

 # This file is used in addition to the one already maintained in Chrome.
 # It acts as a place holder for future additions for WebRTC.
 # It must exist for the Python wrapper script to work properly.
 
 # There are three kinds of suppressions in this file.
 # 1. third_party libraries
 # 2. webrtc stuff
 # 3. libjingle stuff (talk folder)
 #-----------------------------------------------------------------------
 
 # third_party libraries
 
 {
    libnss_leak_hitting_trusty_machines
    Memcheck:Leak
    fun:calloc
    obj:/usr/lib/x86_64-linux-gnu/libnss3.so
    ...
    fun:NSS_NoDB_Init
    ...
 }
 {
    libc_free_hitting_gce_vms
    Memcheck:Free
    fun:free
    fun:__libc_freeres
    fun:_vgnU_freeres
    ...
 }
+{
+   Ubuntu_Precise_Itoa_Optimized_Code
+   Memcheck:Uninitialized
+   fun:_itoa_word
+   fun:vfprintf
+   fun:vsnprintf
+   fun:snprintf
+   ...
+}
 
 #-----------------------------------------------------------------------
 # webrtc stuff
 {
    bug_3446
    Memcheck:Uninitialized
    fun:vp8cx_pick_filter_level_fast
    fun:vp8_loopfilter_frame
    fun:encode_frame_to_data_rate
    fun:vp8_get_compressed_data
@@ skipping 373 matching lines @@
    fun:silk_NLSF_encode
    fun:silk_process_NLSFs
    fun:silk_process_NLSFs_FLP
    fun:silk_find_pred_coefs_FLP
    fun:silk_encode_frame_FLP
    fun:silk_Encode
    fun:opus_encode_native
    fun:opus_encode
    fun:WebRtcOpus_Encode
 }
+{
+   bug_5983
+   Memcheck:Uninitialized
+   fun:_ZN6webrtc11DataChannel4InitERKNS_23InternalDataChannelInitE
+   ...
+}
+{
+   bug_5984
+   Memcheck:Uninitialized
+   fun:_ZN6webrtc15GainControlImpl19ProcessCaptureAudioEPNS_11AudioBufferEb
+   ...
+}