Replace sk_throw with LOG_ALWAYS_FATAL.

The sk_throw macro is now just defined to be SK_ABORT, which is designed
to crash Skia. Replacing sk_throw with LOG_ALWAYS_FATAL should be both
more idomatic and more functional.

In addition this replaces SkASSERT with ALOG_ASSERT in the one file
affected.

Test: refactoring CL. Existing unit tests still pass.
Change-Id: I4e0bd382d9da27e08febec487b43cf83537dce12