Set JAVAC_WRAPPER if USE_GOMA is set.

Now, people can also use goma for building javac.  Let me make it by
default.

Test: USE_GOMA=true m -j4
Bug: 62334576
Change-Id: I941ce42dc0f9cbd0dcc00e6fd19368c3af11695f
diff --git a/core/config.mk b/core/config.mk
index 5ff582a..5895128 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -308,6 +308,7 @@
 
 export CC_WRAPPER
 export CXX_WRAPPER
+export JAVAC_WRAPPER
 endif
 
 ifdef TARGET_PREFER_32_BIT
diff --git a/core/goma.mk b/core/goma.mk
index 4e8318a..2fb37a7 100644
--- a/core/goma.mk
+++ b/core/goma.mk
@@ -45,6 +45,7 @@
   # use both ccache and gomacc.
   CC_WRAPPER := $(strip $(CC_WRAPPER) $(GOMA_CC))
   CXX_WRAPPER := $(strip $(CXX_WRAPPER) $(GOMA_CC))
+  JAVAC_WRAPPER := $(strip $(JAVAC_WRAPPER) $(GOMA_CC))
 
   # gomacc can start goma client's daemon process automatically, but
   # it is safer and faster to start up it beforehand. We run this as a