flag imageinfo as srgb

intended uses:
- flag a SkSurface as sRGB (only supported by Ganesh for now)
- flag images (e.g. png or jpeg) as sRGB if the codec tells us that

wins:
- faster gamma-correct text (esp. w/ distance-fields) when we can use sRGB for text
- better color fidelity when the screen really is sRGB

Review URL: https://codereview.chromium.org/676883003
4 files changed