If maxLength is larger than String::kMaxLength, we used to throw
immediately. However, we must first look at the filler argument, which
is observable. Moreover, if the filler is empty, we must return the
input unchanged.

Bug: v8:8078
Change-Id: Ic3d135f9e25da56df45b059144e45e19dda9c3d8
Reviewed-on: https://chromium-review.googlesource.com/1188313
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55414}