Make BitrateAllocator::transmission_max_bitrate_multiplier_ const.

To make this work, also make the private method
GetTransmissionMaxBitrateMultiplier static.

Bug: None
Change-Id: I1777602b89d404ecbc9956acf9a9c7f852d69911
Reviewed-on: https://webrtc-review.googlesource.com/72440
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23023}
diff --git a/call/bitrate_allocator.cc b/call/bitrate_allocator.cc
index 7a84f25..a300f6d 100644
--- a/call/bitrate_allocator.cc
+++ b/call/bitrate_allocator.cc
@@ -72,6 +72,7 @@
                            num_pause_events_);
 }
 
+// static
 uint8_t BitrateAllocator::GetTransmissionMaxBitrateMultiplier() {
   uint64_t multiplier = strtoul(webrtc::field_trial::FindFullName(
                                     "WebRTC-TransmissionMaxBitrateMultiplier")