Printing: Convert PrintingContext into an interface implemented by the separate
platforms.

BUG=none
TEST=none

Review URL: http://codereview.chromium.org/3610013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@61714 0039d316-1c4b-4281-b951-d872f2087c98

• chrome/browser/printing/print_job_unittest.cc[diff]
• chrome/browser/printing/print_job_worker.cc[diff]
• chrome/browser/printing/print_job_worker.h[diff]
• printing/emf_win_unittest.cc[diff]
• printing/printing.gyp[diff]
• printing/printing_context.cc[Added - diff]
• printing/printing_context.h[diff]
• printing/printing_context_cairo.cc[diff]
• printing/printing_context_cairo.h[Added - diff]
• printing/printing_context_mac.h[Added - diff]
• printing/printing_context_mac.mm[diff]
• printing/printing_context_win.cc[diff]
• printing/printing_context_win.h[Added - diff]
• printing/printing_context_win_unittest.cc[diff]
• printing/printing_test.h[diff]