This was crashing for example here:

https://chromium-swarm.appspot.com/task?id=4b174a9710848410
https://chromium-swarm.appspot.com/task?id=4b214baec2ee7e10

With this stack:

libglesv2!gl::TextureState::isBoundAsSamplerTexture
libglesv2!gl::Texture::onUnbindAsSamplerTexture
libglesv2!gl::State::unsetActiveTexture+0x9
libglesv2!gl::State::updateActiveTextureState+0x2d2
libglesv2!gl::State::updateActiveTexture+0x3aa
libglesv2!gl::State::setSamplerTexture+0x4a1
libglesv2!gl::Context::bindTexture+0x1ab
libglesv2!gl::BindTexture+0x99
chrome!GrGLFunction+0x1f
chrome!GrGLGpu::bindTexture+0x1a0
chrome!GrGLProgram::bindTextures+0x1b9
chrome!GrGLOpsRenderPass::onBindTextures+0x50
chrome!GrOpsRenderPass::bindTextures+0x106
chrome!GrOpFlushState::bindTextures+0xf
chrome!`anonymous namespace'::FillRectOp::onExecute+0xd3

It's unclear how we could end up with a Texture bound that doesn't go
through the normal setter functions. I did see a potential hole where
textures might not get an unbind call when a Context is torn down. This
could lead to bugs in multi-context situations.

This protects the set/unset functions in a helper class to ensure we
always call onBind/onUnbind and forces the destructor to call unbind.

Bug: angleproject:4490
Change-Id: Ied64e02bbe3a37efcab6cbdd4bf2d1b6dcb8b3ac
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2118254
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>