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This replaces usage of NPObject in pepper with gin-backed V8 objects. It is unfortunate that this CL is so large, but there isn't a nice way to have the old implementation and the new one side-by-side.

There are 4 major parts to this CL:
1) Changing the HostVarTracker to track V8ObjectVars rather than NPObjectVars (host_var_tracker.cc).
2) Changing plugin elements (in plugin_object.cc) to be gin-backed objects.
3) Changing postMessage bindings (message_channel.cc) be gin-backed objects.
4) Changing the implementation of PPB_Var_Deprecated (ppb_var_deprecated_impl.cc) to call directly into V8.

BUG=351636

Committed: https://chromium.googlesource.com/chromium/src/+/21f446ae855d60cc896b40cb9a3249ed07f150b3

Committed: https://chromium.googlesource.com/chromium/src/+/ee49e63baf57e503bd71dfe61c8a80df63eac9aa

Review URL: https://codereview.chromium.org/459553003

Cr-Commit-Position: refs/heads/master@{#292823}