Adding subviews in didMoveToSuperview instead
Adding subviews in didMoveToSuperview instead

Adding UIWebView in willMoveToSuperview is causing weird
layout issues to occur.
didMoveToSuperview is called in the same runloop and
immediately after |willMoveToSuperview|. So the goal
of not adding subviews in |init| but before layouting
is still achieved.

BUG=568097

Review URL: https://codereview.chromium.org/1558503002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#367345}

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