Make the snapshot a public dependency of v8.

Make the snapshot a public dependency of v8.

Adds a chain of public dependencies from the v8 target to the snapshot target.
A future version of GN will validate that any files taken as inputs to a step
were generated by public dependencies of that target. Some targets in Chrome
depend on the results of the snapshot.

Review URL: https://codereview.chromium.org/1138953007

Cr-Commit-Position: refs/heads/master@{#28478}