[Fetch API] Keep track of inflight bytes for requests with keepalive set

This change also fixes a web platform test:

 - Wait for request completion by adding text() calls.
 - Fix incorrect promise usage which led to incorrect results.

Bug: 695939
Change-Id: I73cc8406ae2a0344327deac3f16738a0b959a4d8
Reviewed-on: https://chromium-review.googlesource.com/826591
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Takeshi Yoshino <tyoshino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#525670}
8 files changed