Safe numeric library added: base/numerics (copied from Chromium)

This copies the contents (unittest excluded) of base/numerics in
chromium to base/numerics in webrtc. Files added:
- safe_conversions.h
- safe_conversions_impl.h
- safe_math.h
- safe_math_impl.h

A really old version of safe_conversions[_impl].h previously existed in
base/, this has been deleted and sources using it have been updated
to include the new base/numerics/safe_converions.h.

This CL also adds a DEPS file to webrtc/base.

NOPRESUBMIT=True
BUG=webrtc:5548, webrtc:5623

Review URL: https://codereview.webrtc.org/1753293002

Cr-Commit-Position: refs/heads/master@{#11907}
diff --git a/webrtc/modules/audio_coding/neteq/neteq_impl.cc b/webrtc/modules/audio_coding/neteq/neteq_impl.cc
index b4cc915..a6666a6 100644
--- a/webrtc/modules/audio_coding/neteq/neteq_impl.cc
+++ b/webrtc/modules/audio_coding/neteq/neteq_impl.cc
@@ -17,7 +17,7 @@
 
 #include "webrtc/base/checks.h"
 #include "webrtc/base/logging.h"
-#include "webrtc/base/safe_conversions.h"
+#include "webrtc/base/numerics/safe_conversions.h"
 #include "webrtc/base/trace_event.h"
 #include "webrtc/common_audio/signal_processing/include/signal_processing_library.h"
 #include "webrtc/modules/audio_coding/codecs/audio_decoder.h"