Add SkWebpCodec, for decoding .webp images.

Based on SkImageDecoder_libwebp.

TODO:
Support YUV? (Longer term - may influence our API for SkImageGenerator)

BUG=skia:3257

Review URL: https://codereview.chromium.org/1044433002
diff --git a/tests/CodexTest.cpp b/tests/CodexTest.cpp
index 080e6e5..5c96990 100644
--- a/tests/CodexTest.cpp
+++ b/tests/CodexTest.cpp
@@ -89,6 +89,11 @@
     // WBMP
     check(r, "mandrill.wbmp", SkISize::Make(512, 512), false);
 
+    // WEBP
+    check(r, "baby_tux.webp", SkISize::Make(386, 395), false);
+    check(r, "color_wheel.webp", SkISize::Make(128, 128), false);
+    check(r, "yellow_rose.webp", SkISize::Make(400, 301), false);
+
     // BMP
     check(r, "randPixels.bmp", SkISize::Make(8, 8), false);