Revert "Switch MHTML generation to the new callbacks."

This reverts commit d3711b44bcb0cfdab5b0189a064c212e0d1d398f.

Reason for revert: suspected of making BrowserCloseManagerWithDownloadsBrowserTest/BrowserCloseManagerWithDownloadsBrowserTest.TestWithDownloadsFromDifferentProfiles/ very flaky on waterfall

Original change's description:
> Switch MHTML generation to the new callbacks.
> 
> BUG=714018
> 
> Change-Id: I33cd6842055c959af25e286b1962c6f902b35971
> Reviewed-on: https://chromium-review.googlesource.com/974284
> Reviewed-by: John Abd-El-Malek <jam@chromium.org>
> Commit-Queue: Avi Drissman <avi@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#545087}

TBR=avi@chromium.org,jam@chromium.org

Change-Id: I0ed09e0d0322de613180e5b7558256fb3aea58bf
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 714018, 825141
Reviewed-on: https://chromium-review.googlesource.com/977981
Reviewed-by: Christos Froussios <cfroussios@chromium.org>
Commit-Queue: Christos Froussios <cfroussios@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545395}
11 files changed