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This is extracted from the WASM + IndexedDB code into a separate CL, for
better reviewability. Serializing WASM modules will add a format to the
value-wrapping logic, pushing the total complexity to the point where
the benefits of having clearly separated lifecycle steps for
IDBValueWrapper exceed the benefits of having a terse API.

Therefore, this CL introduces a clear separation between the stages of
an IDBValueWrapper's lifecycle, which are:
1) Cloning (a structured clone may be needed to compute index values)
2) Wrapping
3) Extracting information for the backend -- the data bytes and the Blob list

The CL also introduces logic for including byte vectors in wrappers,
which will be needed for the V8 snapshot version -- details in
https://docs.google.com/document/d/1IgUeRLza0WC3uAzXxla5rH1bs5jUHKbLjsfU--zdYag

Last, this CL makes it possible to call
IDBValueWrapper::WrapIfBiggerThan() multiple times, which will come in
handy for the new wrapping format -- details in
https://crrev.com/c/709596/17/third_party/WebKit/Source/modules/indexeddb/IDBValueWrapping.cpp


Bug: 719007
Change-Id: Ie74ddfd4ec849d4777ab9a3af5dc254fc5b14837
Reviewed-on: https://chromium-review.googlesource.com/804969
Commit-Queue: Victor Costan <pwnall@chromium.org>
Reviewed-by: Joshua Bell <jsbell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522369}