This is a reland of 1f1eb625, the
breakage on the GCStress bot seems to be unrelated (maybe flushed
out by this change). We decided to reland to figure out whether it's
a random flake or really triggered by this particular change.

Original change's description:
> [objects] Change String::length field to uint32_t.
>
> This changes the Name::hash_field and Symbol::flags to uint32_t as
> well, so that both Symbols and Strings consume one fewer word on 64-bit
> architectures now. More importantly the access to String::length is
> always a 32-bit field load now, even with 31-bit Smis (i.e. on ARM or
> on 64-bit with pointer compression), so the access should be faster.
>
> Bug: v8:7065, v8:8171
> Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng;luci.v8.try:v8_linux_noi18n_rel_ng
> Change-Id: I5523deb1f84ece91fa2fea775d50318bd1300493
> Reviewed-on: https://chromium-review.googlesource.com/1221288
> Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
> Reviewed-by: Yang Guo <yangguo@chromium.org>
> Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
> Reviewed-by: Igor Sheludko <ishell@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#55825}

Bug: v8:7065, v8:8171
Tbr: tebbi@chromium.org, yangguo@chromium.org, ishell@chromium.org, ulan@chromium.org
Change-Id: I2be24ac018591c04c826e7e8db82e007b738d156
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng;luci.v8.try:v8_linux_noi18n_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/1222308Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55838}