Use access_mode in argument in a constructor of IsolateHolder.

gin::IsolateHolder has 3 types of constructors, and one of them
igored an argument.
This CL makes it to use |access_mode| in the argument.

BUG=None

Review-Url: https://codereview.chromium.org/2709473002
Cr-Commit-Position: refs/heads/master@{#451583}
1 file changed