When changing uniforms of a program, State::onActiveTextureChange is
called to update the ActiveTextureCache. If the sampler uniform type
changes to TextureType::InvalidEnum, the entry in ActiveTextureCache was
not cleared. This causes stale entries in ActiveTextureCache because the
cache no longer matches what textures are bound and the cache does not
add references to the textures in it.

BUG=chromium:1078375
BUG=chromium:1072406
BUG=chromium:1078866

Change-Id: If9719dcd4fc865b2301db450eb8115e7cfe46c4a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2199654Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Geoff Lang <geofflang@chromium.org>