Refactor IsolateHolder to be able to always create the isolate

Currently, blink creates its own isolates. We want to always use an
IsolateHolder to create the isolates. To be able to do this, I introduce
an Initialize method that setups V8. The new IsolateHolder ctor now
doesn't take any parameters but just creates new isolates according to
the configuration.

All non-blink gin users are cut over to the new API

BUG=none
R=abarth@chromium.org,andrewhayden@chromium.org,eroman@chromium.org

Review URL: https://codereview.chromium.org/553903003

Cr-Commit-Position: refs/heads/master@{#294262}

• gin/array_buffer.h[diff]
• gin/isolate_holder.cc[diff]
• gin/public/isolate_holder.h[diff]
• gin/shell/gin_main.cc[diff]
• gin/shell_runner_unittest.cc[diff]
• gin/test/file_runner.cc[diff]
• gin/test/v8_test.cc[diff]
• mojo/apps/js/main.cc[diff]
• mojo/apps/js/test/js_to_cpp_unittest.cc[diff]
• net/DEPS[diff]
• net/proxy/proxy_resolver_v8.cc[diff]