Port session_backend.cc to Posix.

* Using FilePath for paths instead of wstring.
* File operations using cross-platform FileStream instead of win32 API.

Review URL: http://codereview.chromium.org/20327

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@9811 0039d316-1c4b-4281-b951-d872f2087c98

• chrome/browser/browser.scons[diff]
• chrome/browser/sessions/base_session_service.cc[diff]
• chrome/browser/sessions/base_session_service.h[diff]
• chrome/browser/sessions/session_backend.cc[diff]
• chrome/browser/sessions/session_backend.h[diff]
• chrome/browser/sessions/session_backend_unittest.cc[diff]
• chrome/browser/sessions/session_service.cc[diff]
• chrome/browser/sessions/session_service.h[diff]
• chrome/browser/sessions/session_service_unittest.cc[diff]
• chrome/browser/sessions/tab_restore_service.cc[diff]
• chrome/chrome.xcodeproj/project.pbxproj[diff]
• chrome/common/temp_scaffolding_stubs.h[diff]