Previously we captured the PC before calling EnsureSpace in
RecordProtectedInstruction. Sometimes EnsureSpace would resize and move
the buffer, which would invalidate the previously captured PC and trip an
assert when writing RelocInfo. With this change, we do not capture the PC
until after we've ensured there's enough space, which ensures the PC will
be valid.

BUG=

Review-Url: https://codereview.chromium.org/2690523003
Cr-Commit-Position: refs/heads/master@{#43202}