Fix gn_migration.gypi after https://codereview.chromium.org/1231283003/

Without this, building 'all' won't create the isolate file, making the isolate
step after the compile step fail.

BUG=98637
TBR=maruel

Review URL: https://codereview.chromium.org/1244443002

Cr-Commit-Position: refs/heads/master@{#339079}
1 file changed