Verify that gin_shell and hello_world.js exist.

This issue: https://code.google.com/p/chromium/issues/detail?id=398549,
was difficult to debug because gin/shell/gin_shell_unittest.cc didn't verify
that its input files existed before forging ahead.

The unit test now verifies that the gin_shell and hello_world.js paths
exist before launching the former and loading the latter.


BUG=398549

Review URL: https://codereview.chromium.org/471623002

Cr-Commit-Position: refs/heads/master@{#289599}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@289599 0039d316-1c4b-4281-b951-d872f2087c98

gin/shell/gin_shell_unittest.cc

@@ -3,7 +3,7 @@
 // found in the LICENSE file.
 
 #include "base/command_line.h"
-#include "base/files/file_path.h"
+#include "base/file_util.h"
 #include "base/path_service.h"
 #include "base/process/launch.h"
 #include "base/strings/string_util.h"
@@ -25,8 +25,13 @@ base::FilePath HelloWorldPath() {
 }
 
 TEST(GinShellTest, HelloWorld) {
-  CommandLine cmd(GinShellPath());
-  cmd.AppendArgPath(HelloWorldPath());
+  base::FilePath gin_shell_path(GinShellPath());
+  base::FilePath hello_world_path(HelloWorldPath());
+  ASSERT_TRUE(base::PathExists(gin_shell_path));
+  ASSERT_TRUE(base::PathExists(hello_world_path));
+
+  CommandLine cmd(gin_shell_path);
+  cmd.AppendArgPath(hello_world_path);
   std::string output;
   ASSERT_TRUE(base::GetAppOutput(cmd, &output));
   base::TrimWhitespaceASCII(output, base::TRIM_ALL, &output);