This reverts commit df6157ae.

Reason for revert: trybots didn't rerun :-/

Original change's description:
> Reland "[objects] Change String::length field to uint32_t."
> 
> 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}
> 
> Tbr: tebbi@chromium.org, yangguo@chromium.org, ishell@chromium.org
> Bug: v8:7065, v8:8171
> Change-Id: I3c7d0b00abb15fa98ab622f9ecd8602fc798cbc3
> 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/1221290
> Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#55836}

TBR=ulan@chromium.org,yangguo@chromium.org,tebbi@chromium.org,ishell@chromium.org,bmeurer@chromium.org

Change-Id: Ieaf3be31166abb02e37370ad846c38fa3d114693
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:7065, v8:8171
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/1222306Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55837}