Layers view: fetch layer textures at appropriate scale

- add scale parameter to GraphicsContextSnapshot::replay()
    and plumb it through the protocol to front-end;
- extract texture management logic into LayerTextureManager;
- request layer images in a scale matching current scale,
    re-rasterize as required.

BUG=

Review URL: https://codereview.chromium.org/376843004

git-svn-id: svn://svn.chromium.org/blink/trunk@177669 bbb929c8-8fbe-4397-9dbb-9b2b20218538