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Adds support to extension_shell and ash_shell to use external V8 snapshot files
in preparation for moving ChromeOS and ChromeCast to use this.

Re-factors the chrome_content_browser_client and
content/shell_content_browser_client to allow more reuse
of the code which opens the V8 external snapshot for child processes
by adding IsolateHolder::OpenV8FilesForChildProcesses.

This does not yet switch ChromeOS to use external V8 snapshot files -
this will be done in follow up CL https://codereview.chromium.org/1019123002.

BUG=421063

Review URL: https://codereview.chromium.org/1019483002

Cr-Commit-Position: refs/heads/master@{#323953}