Previously this information was encoded in a FixedArray dangling off the
Code object. This extra field seems to be responsible for increased memory
usage, as seen in the linked bugs. In this change, we instead encode this
in the RelocInfo and remove the field from the Code object.

BUG= https://bugs.chromium.org/p/chromium/issues/detail?id=678583
BUG= https://bugs.chromium.org/p/chromium/issues/detail?id=671180
BUG= https://bugs.chromium.org/p/chromium/issues/detail?id=670733

Review-Url: https://codereview.chromium.org/2651833003
Cr-Commit-Position: refs/heads/master@{#42802}