In SkSL, replace 'sample' with '.eval'

SkSL syntax is evolving to better communicate how the effects integrate
with the rest of the Skia pipeline. 'sample' suggests texture sampling,
but the invocation of the shader object is really a call to a function
(the entry point of the SkShader's generated code).

Bug: skbug.com/12302
Change-Id: Ib6b895208ae70368901965b7ca13434c51a7455f