Base the fast-path in AdjustAmountOfExternalMemory on a value + limit. To
preserve the behavior the limit is just set using kExternalAllocationLimit.

Redo naming of related members.

R=jochen@chromium.org
BUG=chromium:621829
LOG=N

Review-Url: https://codereview.chromium.org/2085893002
Cr-Commit-Position: refs/heads/master@{#37131}