The DCHECK verified capacity just before the call to EnsureCapacity()
(which extends capacity if needed). This DCHECK can just be removed
since FixedArray::set() already checks the given index is in-bounds.

Drive-by: Remove similar duplicate DCHECKs in FixedArrayBuilder.

Bug: chromium:933776
Change-Id: I9f058548063a170ea6dce112a3877792887efcca
Reviewed-on: https://chromium-review.googlesource.com/c/1479955Reviewed-by: Simon Zünd <szuend@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59753}