Send captured content per document

- Moves the session related code to TaskSession.
- TaskSession categorizes the captured content and the detached
  nodes into DocumentSession.
- ContentCaptureTask uses the DocumentSession to send the data
  though the LocalFrame's WebContentCaptureClient.

Bug: 924681
Change-Id: I8cd5c96c1ab183e7a567a70fef813881c8b2ab4a
Reviewed-on: https://chromium-review.googlesource.com/c/1493400
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Tao Bai <michaelbai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636680}
10 files changed