Distinguish the compilation caches instead by the shape of the key (cow fixed
array map meaning eval or script cache). This allows us to remove the odd "key"
argument from Shrink, EnsureCapacity and Rehash.

Bug: v8:6474
Change-Id: Ibcad22813063c3a9050da13dc51359f5b59e1254
Reviewed-on: https://chromium-review.googlesource.com/531184Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45873}