Add enum value ScaleMethod::kFilterLinear.

Needed to get the same numerical values as used in libyuv.

BUG=
R=ossu@webrtc.org, pbos@webrtc.org

Review URL: https://codereview.webrtc.org/2033143003 .

Cr-Commit-Position: refs/heads/master@{#13035}

webrtc/common_video/libyuv/include/scaler.h

@@ -23,8 +23,10 @@
 namespace webrtc {
 
 // Supported scaling types
+// Note: Must have the same values as libyuv::FilterMode.
 enum ScaleMethod {
   kScalePoint,  // no interpolation
+  kFilterLinear,
   kScaleBilinear,
   kScaleBox
 };