We can't assume that there's no entered context during microtask
execution. We also can't assume that we'll never enter an context during
microtask execution either.

We can however assume that we don't execute microtasks recursively, so
remembering the length of the entered context stack is enough to decide
which context to use.

BUG=chromium:635769
R=verwaest@chromium.org

Review-Url: https://codereview.chromium.org/2350933007
Cr-Commit-Position: refs/heads/master@{#39627}