Use text/plain as a test MIME type instead of application/pdf for NetUtilTest.GenerateFileName.

application/pdf may not have a preferred extension mapping on all bots.

BUG=NetUtilTest.GenerateFileName test fails on Win Beta builder.
TEST=net_unittests --gtest_filter=NetUtilTest.GenerateFileName

Review URL: http://codereview.chromium.org/8392039

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@107756 0039d316-1c4b-4281-b951-d872f2087c98

net/base/net_util_unittest.cc

@@ -2289,16 +2289,16 @@ TEST(NetUtilTest, GenerateFileName) {
       "",
       "",
       "",
-      "application/pdf",
+      "text/plain",
       L"download",
-      L"npdf.pdf"
+      L"npdf" TXT_EXT
     },
     { // Shouldn't overwrite C-D specified extension.
       "http://www.example.com/npdf.php?fn=foobar.pdf",
       "Content-Disposition: filename=foobar.jpg",
       "",
       "",
-      "application/pdf",
+      "text/plain",
       L"download",
       L"foobar.jpg"
     },