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Even though it's having WebContents without a ContentViewCore
is a support scenario in //content, it's not a well tested
path, especially the webview only code. This makes sure
ContentViewCore and WebContents are destroyed together.

Instead of posting on native side to destroy WebContents,
post on Java side and have one call destroy both Java and
native side.

More specific details:
* All caller except destroy to AwContents.destroyNatives
  either have already detached webview so ok to move detach
  to destroy.
* Need to remove some asserts that objects are null when
  mIsDestroyed is true.
* Reordered some native AwContents members so they are
  naturally destroyed in the correct order.

BUG=

Review URL: https://codereview.chromium.org/690553002

Cr-Commit-Position: refs/heads/master@{#302513}