Fix "implicit conversion loses integer precision" in linux dbg 32 bot

Link to the failure: [1].

[1] https://ci.chromium.org/p/chromium/builders/ci/Linux%20Builder%20%28dbg%29%2832%29/118453

Bot is not on the waterfall, hence, CQ did not notice it.
CL that originally caused it: [2].

[2] https://crrev.com/c/1775002

BUG=787254
R=guidou@chromium.org, haraken@chromium.org

Change-Id: I5f33b1b1f2a0c7e48d74af99e4a333805ca8569f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1775869
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Auto-Submit: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#691640}

third_party/blink/renderer/platform/peerconnection/rtc_video_encoder.cc

@@ -87,7 +87,7 @@ bool GetRTPFragmentationHeaderH264(webrtc::RTPFragmentationHeader* header,
   header->VerifyAndAllocateFragmentationHeader(nalu_vector.size());
   for (size_t i = 0; i < nalu_vector.size(); ++i) {
     header->fragmentationOffset[i] = nalu_vector[i].data - data;
-    header->fragmentationLength[i] = nalu_vector[i].size;
+    header->fragmentationLength[i] = static_cast<size_t>(nalu_vector[i].size);
   }
   return true;
 }