Forward declaration for Isolate / Platform in libplatform.h.

Reduces dependencies on #include files, making it easier for other
build systems to include this library.

BUG=

Review URL: https://codereview.chromium.org/740493002

Cr-Commit-Position: refs/heads/master@{#25408}
Fixes d8 on windows following.

Broke by https://codereview.chromium.org/740493002/

BUG=
NOTRY=true
NOTREECHECKS=true

Review URL: https://codereview.chromium.org/734853004

Cr-Commit-Position: refs/heads/master@{#25409}