The test does this:

1. Context 1: Read Texture 1 and draw into Framebuffer 1
2. Context 2: Read Texture 1 and draw into Framebuffer 2
3. Context 1: Delete Framebuffer 1
4. Context 1: Flush
5. Context 2: Modify Texture 1

Issue is Texture 1's mCurrentReadingNodes contains one node from each
context's command graph, one of which is deleted at step 4. At step 5, a
dependency is added from both nodes (one already deleted) to a new node,
causing use-after-free.

Bug: angleproject:4130
Change-Id: I06720aec20d0b49114937f1cd9b193a4f1df9d8d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1924790Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>