Use text/plain as a test MIME type instead of application/pdf for NetUtilTest.GenerateFileName.
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: de016e52bd170d2d4f2344f9bf92d50478b649e0@107756 0039d316-1c4b-4281-b951-d872f2087c98

gavinp/chromium,yitian134/chromium,adobe/chromium,yitian134/chromium,ropik/chromium,gavinp/chromium,adobe/chromium,ropik/chromium,adobe/chromium,adobe/chromium,yitian134/chromium,gavinp/chromium,gavinp/chromium,gavinp/chromium,gavinp/chromium,adobe/chromium,ropik/chromium,gavinp/chromium,adobe/chromium,adobe/chromium,adobe/chromium,yitian134/chromium,yitian134/chromium,yitian134/chromium,adobe/chromium,ropik/chromium,yitian134/chromium,ropik/chromium,gavinp/chromium,gavinp/chromium,yitian134/chromium,yitian134/chromium,adobe/chromium,ropik/chromium,adobe/chromium,ropik/chromium,ropik/chromium,yitian134/chromium,gavinp/chromium,ropik/chromium