Make stencil buffers uncached for uncached render target textures

Make stencil buffers uncached for uncached render target textures

Make new stencil buffers of uncached render target textures not affect the
cache budgets. This is consistent with render buffer storage of uncached
render target textures.

Affects only newly created stencil buffers. An uncached render target
might still receive a cached stencil buffer if such is available from
cache.

BUG=skia:3119
BUG=skia:3301

Review URL: https://codereview.chromium.org/859013002