Modify PnaclHeaderTest to run w/ an empty PNaCl component dir.

This helps check that the pexe fetch is able to happen before
the component files are accessed. Otherwise, if the component
files are accessed before the pexe fetch, then it will error
out early, and the PNaCl on-demand installation will not happen.

Followup to Justin's fix: https://codereview.chromium.org/453073002
I tested that without Justin's fix, this patch will make the test catch the error.

BUG=401755

Review URL: https://codereview.chromium.org/467863002

Cr-Commit-Position: refs/heads/master@{#289182}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@289182 0039d316-1c4b-4281-b951-d872f2087c98

chrome/test/nacl/pnacl_header_test.cc

@@ -6,6 +6,7 @@
 
 #include "base/bind.h"
 #include "base/path_service.h"
+#include "base/test/scoped_path_override.h"
 #include "chrome/browser/ui/browser.h"
 #include "chrome/browser/ui/tabs/tab_strip_model.h"
 #include "chrome/common/chrome_paths.h"
@@ -45,6 +46,16 @@ void PnaclHeaderTest::RunLoadTest(const std::string& url,
   content::JavascriptTestObserver observer(
       browser()->tab_strip_model()->GetActiveWebContents(),
       &handler);
+
+  // Make sure this is able to do a pexe fetch, even without access
+  // to the PNaCl component files (make DIR_PNACL_COMPONENT empty).
+  // The pexe fetch that is done with special headers must be able to
+  // start before the component files are on disk. This is because it
+  // is the pexe fetch that helps trigger an on-demand installation
+  // which installs the files to disk (if that hasn't already happened
+  // in the background).
+  base::ScopedPathOverride component_dir(chrome::DIR_PNACL_COMPONENT);
+
   ui_test_utils::NavigateToURL(browser(), embedded_test_server()->GetURL(url));
   // Wait until the NMF and pexe are also loaded, not just the HTML.
   // Do this by waiting till the LoadTestMessageHandler responds.