Add webp to filtered drawable resources.

Summary:
This way webp in drawable per-dpi directories will be filtered too
(and we will be able to use webp for newer phones).

src/com/facebook/buck/android/FilterResourcesStep.java

diff --git a/src/com/facebook/buck/android/FilterResourcesStep.java b/src/com/facebook/buck/android/FilterResourcesStep.java
index 04957c3..8b3cf91 100644
--- a/src/com/facebook/buck/android/FilterResourcesStep.java
+++ b/src/com/facebook/buck/android/FilterResourcesStep.java
@@ -62,7 +62,7 @@
 
   @VisibleForTesting
   static final Pattern DRAWABLE_PATH_PATTERN = Pattern.compile(
-      ".*drawable.*/.*(png|jpg|jpeg|gif)", Pattern.CASE_INSENSITIVE);
+      ".*drawable.*/.*(png|jpg|jpeg|gif|webp)", Pattern.CASE_INSENSITIVE);
 
   @VisibleForTesting
   static final Pattern NON_ENGLISH_STRING_PATH = Pattern.compile(