Send encoded_body_length to renderer when response completed (1/3)

Currently, "encoded_body_length" is dispatched on each ResourceMsg_DataReceived
IPC message. With this series of CLs, it will be dispatched when a resource
loading is completed. This is needed to use mojo data pipe for transferring
the response body.

 - 1/3: https://codereview.chromium.org/2514713002/
 - 2/3: https://codereview.chromium.org/2510333002/
 - 3/3: https://codereview.chromium.org/2506363005/

Design doc: https://docs.google.com/document/d/1Q-LpTPcV8QApeXSL24MVZHjP-uQK7RAK300t_qZaGFQ/edit#

This CL adds "encoded_body_length" property to ResourceRequestCompletionStatus.
This CL doesn't change the behavior.

BUG=666216

Review-Url: https://codereview.chromium.org/2514713002
Cr-Commit-Position: refs/heads/master@{#434032}
9 files changed