The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b647f8362016d8e5b7a614e92b72331d070a32cf

commit b647f8362016d8e5b7a614e92b72331d070a32cf
Author: liaoyuke <liaoyuke@chromium.org>
Date: Fri Apr 14 23:52:03 2017

Replace requirePageReconstruction with setCustomUserAgent

WKWebView provides a property: customUserAgent that we can use to
change the user agent associated with the web view, which means that we
don't have to requirePageConstruction anymore when user agent changes.

After applying this change, I observed that it's becoming significantly
faster to navigate between desktop and mobile pages during back and
forward navigations.

BUG= 707368 

Review-Url: https://codereview.chromium.org/2811073005
Cr-Commit-Position: refs/heads/master@{#464830}

[modify] https://crrev.com/b647f8362016d8e5b7a614e92b72331d070a32cf/ios/chrome/browser/tabs/tab.mm
[modify] https://crrev.com/b647f8362016d8e5b7a614e92b72331d070a32cf/ios/web/web_state/ui/crw_web_controller.h
[modify] https://crrev.com/b647f8362016d8e5b7a614e92b72331d070a32cf/ios/web/web_state/ui/crw_web_controller.mm
[modify] https://crrev.com/b647f8362016d8e5b7a614e92b72331d070a32cf/ios/web/web_state/web_view_internal_creation_util.h
[modify] https://crrev.com/b647f8362016d8e5b7a614e92b72331d070a32cf/ios/web/web_state/web_view_internal_creation_util.mm
[modify] https://crrev.com/b647f8362016d8e5b7a614e92b72331d070a32cf/ios/web/web_view_creation_util.mm

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/57f76e7371706d96c3cd0915f8efc04fab8c1166

commit 57f76e7371706d96c3cd0915f8efc04fab8c1166
Author: rohitrao <rohitrao@chromium.org>
Date: Mon Apr 17 12:54:42 2017

Revert of Replace requirePageReconstruction with setCustomUserAgent (patchset #5 id:100001 of https://codereview.chromium.org/2811073005/ )

Reason for revert:
Seeing crashes in EG tests: https://build.chromium.org/p/chromium.fyi/builders/EarlGreyiOS/builds/26550

Original issue's description:
> Replace requirePageReconstruction with setCustomUserAgent
>
> WKWebView provides a property: customUserAgent that we can use to
> change the user agent associated with the web view, which means that we
> don't have to requirePageConstruction anymore when user agent changes.
>
> After applying this change, I observed that it's becoming significantly
> faster to navigate between desktop and mobile pages during back and
> forward navigations.
>
> BUG= 707368 
>
> Review-Url: https://codereview.chromium.org/2811073005
> Cr-Commit-Position: refs/heads/master@{#464830}
> Committed: https://chromium.googlesource.com/chromium/src/+/b647f8362016d8e5b7a614e92b72331d070a32cf

TBR=eugenebut@chromium.org,kkhorimoto@chromium.org,liaoyuke@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG= 707368 

Review-Url: https://codereview.chromium.org/2820983003
Cr-Commit-Position: refs/heads/master@{#464917}

[modify] https://crrev.com/57f76e7371706d96c3cd0915f8efc04fab8c1166/ios/chrome/browser/tabs/tab.mm
[modify] https://crrev.com/57f76e7371706d96c3cd0915f8efc04fab8c1166/ios/web/web_state/ui/crw_web_controller.h
[modify] https://crrev.com/57f76e7371706d96c3cd0915f8efc04fab8c1166/ios/web/web_state/ui/crw_web_controller.mm
[modify] https://crrev.com/57f76e7371706d96c3cd0915f8efc04fab8c1166/ios/web/web_state/web_view_internal_creation_util.h
[modify] https://crrev.com/57f76e7371706d96c3cd0915f8efc04fab8c1166/ios/web/web_state/web_view_internal_creation_util.mm
[modify] https://crrev.com/57f76e7371706d96c3cd0915f8efc04fab8c1166/ios/web/web_view_creation_util.mm

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b1688d0107c311a09b0bfb03df0b81ab9b21afc0

commit b1688d0107c311a09b0bfb03df0b81ab9b21afc0
Author: liaoyuke <liaoyuke@chromium.org>
Date: Mon Apr 17 22:18:18 2017

Re-land "Replace requirePageReconstruction with setCustomUserAgent"

This CL re-lands the "Replace requirePageReconstruction with
setCustomUserAgent" CL.

The following is the original CL descriptions:

WKWebView provides a property: customUserAgent that we can use to
change the user agent associated with the web view, which means that we
don't have to requirePageConstruction anymore when user agent changes.

After applying this change, I observed that it's becoming significantly
faster to navigate between desktop and mobile pages during back and
forward navigations.

BUG= 707368 

Review-Url: https://codereview.chromium.org/2822003002
Cr-Commit-Position: refs/heads/master@{#465042}

[modify] https://crrev.com/b1688d0107c311a09b0bfb03df0b81ab9b21afc0/ios/chrome/browser/tabs/tab.mm
[modify] https://crrev.com/b1688d0107c311a09b0bfb03df0b81ab9b21afc0/ios/web/web_state/ui/crw_web_controller.h
[modify] https://crrev.com/b1688d0107c311a09b0bfb03df0b81ab9b21afc0/ios/web/web_state/ui/crw_web_controller.mm
[modify] https://crrev.com/b1688d0107c311a09b0bfb03df0b81ab9b21afc0/ios/web/web_state/web_view_internal_creation_util.h
[modify] https://crrev.com/b1688d0107c311a09b0bfb03df0b81ab9b21afc0/ios/web/web_state/web_view_internal_creation_util.mm
[modify] https://crrev.com/b1688d0107c311a09b0bfb03df0b81ab9b21afc0/ios/web/web_view_creation_util.mm