ToDirectStringAssembler::PointerToData returns a raw pointer, which
is invalidated when GC moves the original string and hence must not
be accessed after any allocations. This fixes the bug introduced in
b4ebbc57 / r53260.

Bug: chromium:845060
Tbr: jgruber@chromium.org
Change-Id: I248d0dd2a275bf9308269b3f65d00c4c4c3d4292
Reviewed-on: https://chromium-review.googlesource.com/1068213
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53272}