With the current approach we cannot eliminate context accesses in
mid-size function contexts, so let's bump the limit a bit to make
sure we can optimize those as well.

R=jarin@chromium.org
BUG=v8:4930,v8:5141

Review-Url: https://codereview.chromium.org/2182973004
Cr-Commit-Position: refs/heads/master@{#38050}