Reserving space for deserialization can cause GC, which
can evict entries from the string table. Having more deleted
entries now, StringTable::EnsureCapacity could cause a GC
later during deserialization even when we actually still
have enough capacity.

Instead, we now keep new internalized strings in a separate list
and commit them to the string table at the end.

R=ulan@chromium.org
BUG=chromium:502085
LOG=N

Review URL: https://codereview.chromium.org/1204863006

Cr-Commit-Position: refs/heads/master@{#29308}