Add Linux breakpad targets and gtest_main to GN build.

tools/gn/secondary/testing/gtest/BUILD.gn

Index: tools/gn/secondary/testing/gtest/BUILD.gn
diff --git a/tools/gn/secondary/testing/gtest/BUILD.gn b/tools/gn/secondary/testing/gtest/BUILD.gn
index 44307fa4362ee91af4e570f49146c195d702461c..623ff6a9b95c829bc95453ba9e3ebbff1fbc9309 100644
--- a/tools/gn/secondary/testing/gtest/BUILD.gn
+++ b/tools/gn/secondary/testing/gtest/BUILD.gn
@@ -102,3 +102,8 @@ static_library("gtest") {
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [ "//build/config/compiler:no_chromium_code" ]
 }
+
+static_library("gtest_main") {

[jamesr, 2014/06/18 18:39:59]

putting a main in a static library is pretty funky, to say the least. should
this be a source_set?

+  sources = [ "src/gtest_main.cc" ]
+  deps = [ ":gtest" ]
+}