New method EncodedImage::set_buffer

This setter method is intended to replace all direct assignments to
the |_size| member. A later cl will make that member private, and
rename it to |capacity_|.

Bug: webrtc:9378
Change-Id: I37e9eb54d1c72bcd4cb8a1cfef34bbc6c209bd0d
Reviewed-on: https://webrtc-review.googlesource.com/c/113060
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25894}

api/video/encoded_image.h

@@ -70,6 +70,11 @@ class RTC_EXPORT EncodedImage {
   size_t size() const { return _length; }
   size_t capacity() const { return _size; }
 
+  void set_buffer(uint8_t* buffer, size_t capacity) {
+    _buffer = buffer;
+    _size = capacity;
+  }
+
   uint32_t _encodedWidth = 0;
   uint32_t _encodedHeight = 0;
   // NTP time of the capture time in local timebase in milliseconds.