Set pointers in internal fields directly instead of wrapping them

Set pointers in internal fields directly instead of wrapping them

The internal fields are scanned by the garbage collector, so they can't contain arbitrary data. However, aligned pointers are supported by the V8 GC, so the V8
API allows for setting a pointer directly instead of wrapping it in an External
container.

Not only is this faster, but it's also required for the new v8::Global API which
I'm going to update to in a follow-up patch.

R=tsepez@chromium.org
BUG=

Committed: https://pdfium.googlesource.com/pdfium/+/2a47d8d6eef22d117ac3475e84c3a52b5ca41a3c

[Tom Sepez, 2015-05-12 15:24:50]

Can we get more information in the CL description about why this change is safe,
e.g. why handles aren't required here any more?  Otherwise LGTM.

[jochen (gone - plz use gerrit), 2015-05-13 00:31:08]

Committed patchset #3 (id:40001) manually as
2a47d8d6eef22d117ac3475e84c3a52b5ca41a3c (presubmit successful).