Add GN output prefix override and allow empty output extensions.

tools/gn/target_unittest.cc

Index: tools/gn/target_unittest.cc
diff --git a/tools/gn/target_unittest.cc b/tools/gn/target_unittest.cc
index b3a85b75a0d824d28bff94e24065c006a8bfb911..3816fe4a314ad1b4be3291cc5b63865d896be4d6 100644
--- a/tools/gn/target_unittest.cc
+++ b/tools/gn/target_unittest.cc
@@ -294,6 +294,13 @@ TEST(Target, GetComputedOutputName) {
   ASSERT_TRUE(dup_prefix.OnResolved(&err));
   EXPECT_EQ("libbar", dup_prefix.GetComputedOutputName(false));
   EXPECT_EQ("libbar", dup_prefix.GetComputedOutputName(true));
+
+  // Target with an output prefix override should not a prefix.

[Dirk Pranke, 2016/04/07 21:43:54]

nit: s/should not a/should not have a/

+  TestTarget override_prefix(setup, "//foo:bar", Target::SHARED_LIBRARY);
+  override_prefix.set_output_prefix_override(true);
+  ASSERT_TRUE(dup_prefix.OnResolved(&err));
+  EXPECT_EQ("bar", override_prefix.GetComputedOutputName(false));
+  EXPECT_EQ("bar", override_prefix.GetComputedOutputName(true));
 }
 
 // Test visibility failure case.