Fix Chromium compile of StereoEncoderAdapter.

WebRTC rolls into Chromium are failing, we should fix it ASAP.

Log:
FAILED:
obj/third_party/webrtc/modules/video_coding/webrtc_stereo/stereo_encoder_adapter.obj
ninja -t msvc -e environment.x64 -- E:\b\c\goma_client/gomacc.exe
"e:\b\c\win_toolchain\vs_files\88c3b62e1eb0893b8cd57e3f4859c3af27907f64\vc\tools\msvc\14.11.25503\bin\hostx64\x64/cl.exe"
/nologo /showIncludes
@obj/third_party/webrtc/modules/video_coding/webrtc_stereo/stereo_encoder_adapter.obj.rsp
/c
../../third_party/webrtc/modules/video_coding/codecs/stereo/stereo_encoder_adapter.cc
/Foobj/third_party/webrtc/modules/video_coding/webrtc_stereo/stereo_encoder_adapter.obj
/Fd"obj/third_party/webrtc/modules/video_coding/webrtc_stereo_cc.pdb"
../../third_party/webrtc/modules/video_coding/codecs/stereo/stereo_encoder_adapter.cc(134):
error C2220: warning treated as error - no 'object' file generated
../../third_party/webrtc/modules/video_coding/codecs/stereo/stereo_encoder_adapter.cc(134):
warning C4267: 'argument': conversion from 'size_t' to 'uint32_t',
possible loss of data

Bug: chromium:780411
Change-Id: Ia80f4551d0efeebc6d084e951f5c25e8b9401250
Reviewed-on: https://webrtc-review.googlesource.com/17781
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Max Morin <maxmorin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20522}
diff --git a/modules/video_coding/codecs/stereo/stereo_encoder_adapter.cc b/modules/video_coding/codecs/stereo/stereo_encoder_adapter.cc
index f6a04d0..2ef0641 100644
--- a/modules/video_coding/codecs/stereo/stereo_encoder_adapter.cc
+++ b/modules/video_coding/codecs/stereo/stereo_encoder_adapter.cc
@@ -130,8 +130,8 @@
     // TODO(emircan): |new_framerate| is used to calculate duration for encoder
     // instances. We report the total frame rate to keep real time for now.
     // Remove this after refactoring duration logic.
-    const int rv =
-        encoder->SetRateAllocation(bitrate, encoders_.size() * framerate);
+    const int rv = encoder->SetRateAllocation(
+        bitrate, static_cast<uint32_t>(encoders_.size()) * framerate);
     if (rv)
       return rv;
   }