Fix GrGradientEffect::isValid()

This was relying on TextureSampler::proxy() which crashes if the proxy is not valid.

Bug: skia:
Change-Id: I8452f5cea3a71b5ced15259aede52a44ab152cf2
Reviewed-on: https://skia-review.googlesource.com/19403
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
2 files changed