Clear document scope dirtiness in import StyleEngine.
This is part of the plan to fix 717506 by re-collect sheets for
style_sheets_for_style_sheet_list_ separately from updating all of
active style to make it more light-weight.
Some sanity checking and comment about document scope dirtiness in,
and clear dirtiness after updating active sheets for, html imports.
Also renamed to UpdateActiveStyleSheetsInImport() to make it clearer
what the method does.
BUG=717506
Review-Url: https://codereview.chromium.org/2880263002
Cr-Commit-Position: refs/heads/master@{#472447}
4 files changed
