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It is now implemented as a proxy object (CRWWebViewScrollViewProxy)
which pretends itself to be a UIScrollView, instead of CWVScrollView.

Define CWVWebView.legacyScrollView to allow to fall back to the original
implementation in runtime temporarily.

Change-Id: I2993ab2ccf78de09412c9f607a74165fea8eae2f
Bug: 1023250
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2137201
Commit-Queue: Hiroshi Ichikawa <ichikawa@chromium.org>
Auto-Submit: Hiroshi Ichikawa <ichikawa@chromium.org>
Reviewed-by: John Wu <jzw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#762718}