[run_web_tests] Combine testRunner.dumpChildFrame* into testRunner.dumpChildFrames()

The original functions dumpChildFrameScrollPositions(),
dumpChildFramesAsText() and dumpChildFramesAsMarkup() did almost the
same thing while each had its own applicable condition.

Combine them to simplify API and prevent wrong usages.

This is a pre-cleanup of making pixel tests not dump text by default
(https://chromium-review.googlesource.com/c/chromium/src/+/1277654).

Bug: 703899
Change-Id: I41adff013a512e6e247b3c12080a71dd3a8ac99f
Reviewed-on: https://chromium-review.googlesource.com/c/1278057
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599553}