The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/97c1dc4aa9e0e242e1b79ffad84806865c782509

commit 97c1dc4aa9e0e242e1b79ffad84806865c782509
Author: rune <rune@opera.com>
Date: Mon Jun 27 08:48:41 2016

Move preferred stylesheet set out of active sheet update.

There are two reasons for this:

1. In preparation for async active stylesheet update, in which case
   setting the preferred stylesheet set would otherwise be happening
   too late.

2. Doing it during the active stylesheet update means only the
   alternate stylesheets following the link with the title setting the
   preferred sheet set would be enabled appropriately.  crbug.com/621479 

This CL is split out of https://codereview.chromium.org/1913833002

R=timloh@chromium.org
BUG= 567021 , 621479 

Review-Url: https://codereview.chromium.org/2079303002
Cr-Commit-Position: refs/heads/master@{#402139}

[add] https://crrev.com/97c1dc4aa9e0e242e1b79ffad84806865c782509/third_party/WebKit/LayoutTests/fast/css/preferred-stylesheet-enabling-alternate.html
[modify] https://crrev.com/97c1dc4aa9e0e242e1b79ffad84806865c782509/third_party/WebKit/Source/core/dom/DocumentStyleSheetCollection.cpp
[modify] https://crrev.com/97c1dc4aa9e0e242e1b79ffad84806865c782509/third_party/WebKit/Source/core/dom/ProcessingInstruction.cpp
[modify] https://crrev.com/97c1dc4aa9e0e242e1b79ffad84806865c782509/third_party/WebKit/Source/core/dom/StyleEngine.cpp
[modify] https://crrev.com/97c1dc4aa9e0e242e1b79ffad84806865c782509/third_party/WebKit/Source/core/dom/StyleSheetCandidate.cpp
[modify] https://crrev.com/97c1dc4aa9e0e242e1b79ffad84806865c782509/third_party/WebKit/Source/core/dom/StyleSheetCandidate.h
[modify] https://crrev.com/97c1dc4aa9e0e242e1b79ffad84806865c782509/third_party/WebKit/Source/core/html/HTMLLinkElement.cpp