Update V8 deps for PRODUCT_PREBUILT_WEBVIEWCHROMIUM.

When using a prebuilt chromium, the build was failing as the set of V8
makefiles being included is no longer correct with the M39 version of
Chromium. Remove generate_trig_table which no longer exists, and add
v8_libplatform which is a new dependency required to build the host
mksnapshot binary.

Bug: 18675666
Change-Id: Ic00eea284a5b40af41883e000e092ee16d6bfcc7
diff --git a/Android.mk b/Android.mk
index 6249a1c..5a873ae 100644
--- a/Android.mk
+++ b/Android.mk
@@ -18,11 +18,11 @@
 ifeq ($(PRODUCT_PREBUILT_WEBVIEWCHROMIUM),yes)
 GYP_MAKEFILE_STEMS := \
 	$(addprefix $(CHROMIUM_DIR)/v8/tools/gyp/, \
-		generate_trig_table.host \
 		js2c.host \
 		mksnapshot.host \
 		v8_base.host \
 		v8_libbase.host \
+		v8_libplatform.host \
 		v8_nosnapshot.host \
 		v8_base.target \
 		v8_libbase.target \