At the moment, the isolate is allocated and initialized in a single
step. This has the downside that the platform cannot register the
isolate before the isolate gets initialized, and therefore the platform
is not available for the isolate during initialization. With this CL we
register the uninitialized isolate on the platform and initialize the
isolate after that.

This change is needed to allow the creation of task runners already
during the initialization of the isolate.

The related chromium CL: https://crrev.com/c/1015020

R=yangguo@chromium.org

Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I52e89388a757f2693d1a800e7aa7701aa0080795
Reviewed-on: https://chromium-review.googlesource.com/1014044Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52731}