Fix *StaticBitmapImage ThreadChecker and unaccelerated SkImage destroy

- AcceleratedStaticBitmapImage was misusing ThreadChecker by having its
own detach logic. Using proper DetachThread is simpler, cleaner and
correct.
- UnacceleratedStaticBitmapImage didn't destroy the SkImage in the
proper thread, leading to GrContext/SkSp problems.

Bug: 890576
Change-Id: Ic71e7f7322b0b851774628247aa5256664bc0723
Reviewed-on: https://chromium-review.googlesource.com/c/1307775
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Fernando Serboncini <fserb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604427}
AcceleratedStaticBitmapImage::ContextProviderWrapper() const {
  if (!IsValid())
    return nullptr;
  return texture_holder_->ContextProviderWrapper();
 }

AcceleratedStaticBitmapImage::ContextProviderWrapper() const {
  if (!IsValid())
    return nullptr;
  return texture_holder_->ContextProviderWrapper();
 }
