Remove unused fields in Blink

This patch removes the following unused fields in Blink:
Document: bool m_loadingSheet
DeleteSelectionCommand: bool m_replace (required callsite cleanup)
IndentOutdentCommand: int m_marginInPixels
MemoryCache: bool m_prePainting
MemoryCache: double m_deadDecodedDataDeletionInterval
HTMLMediaElement: unsigned m_previousProgress
HTMLTokenizer::Checkpoint: bool forceNullCharacterReplacement
EventHandler: bool m_panScrollButtonPressed
GraphicsContext3D: ResourceSafety m_resourceSafety

This represents all but 3 of the unused fields in Blink. I'll
submit a followup to get the remaining ones.

Review URL: https://codereview.chromium.org/116443006

git-svn-id: svn://svn.chromium.org/blink/trunk@164025 bbb929c8-8fbe-4397-9dbb-9b2b20218538