Watch out for SkFixed overflow in SkMipMap.cpp.

Tested with -fsanitize=signed-integer-overflow.
This new assert used to trigger in MipMap unit test.

Don't appear to be any GM diffs.

BUG=skia:

Review URL: https://codereview.chromium.org/729373004
Simplfy DEPS: always pull all deps, remove unused barelinux platform.

Left some notes about where we might further clean up.

BUG=skia:

Review URL: https://codereview.chromium.org/753633002
Switch SkBitmapProcState's SkFractionalInt to 32.32.

16 bits are not enough to hold the integral part of fx in
Clamp_S32_opaque_D32_nofilter_DX_shaderproc.

Weirdly, no GM diffs on my desktop.

BUG=skia:3096

Review URL: https://codereview.chromium.org/733163003
SkPicture::SnapshotArray doesn't really stand to benefit from refcounting.

BUG=skia:

Review URL: https://codereview.chromium.org/748803005
ack verylargebitmap diffs

NOTRY=true

BUG=skia:

Review URL: https://codereview.chromium.org/752693002