Add platform/external/libwebm.

Bug: 173733670
Change-Id: I4f03213a578057978cf162f9216e3a601d67c5e2
diff --git a/default.xml b/default.xml
index a376ab5..b3698c1 100644
--- a/default.xml
+++ b/default.xml
@@ -308,6 +308,7 @@
   <project path="external/libutf" name="platform/external/libutf" groups="pdk" />
   <project path="external/libvpx" name="platform/external/libvpx" groups="pdk" />
   <project path="external/libvterm" name="platform/external/libvterm" groups="pdk" />
+  <project path="external/libwebm" name="platform/external/libwebm" groups="pdk" />
   <project path="external/libwebsockets" name="platform/external/libwebsockets" groups="pdk" />
   <project path="external/libxaac" name="platform/external/libxaac" groups="pdk" />
   <project path="external/libxkbcommon" name="platform/external/libxkbcommon" groups="pdk" />