Calculate size of new data in SetData() for GC accounting

The previous code was using data_->size() for both
|external_allocated_size_| and |old_external_allocated_size|,
which would lead to AdjustAmountOfExternalAllocatedMemory()
always being passed 0.

Bug: None
Change-Id: If50fbc029124237849121aa56b49093e0890ec85
Reviewed-on: https://chromium-review.googlesource.com/1180467
Commit-Queue: Chase Phillips <cmp@chromium.org>
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584262}
1 file changed