Revert of Replace requirePageReconstruction with setCustomUserAgent

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}
Committed: https://chromium.googlesource.com/chromium/src/+/57f76e7371706d96c3cd0915f8efc04fab8c1166

[rohitrao (ping after 24h), 2017-04-17 12:43:20]

The CQ bit was checked by rohitrao@chromium.org

[rohitrao (ping after 24h), 2017-04-17 12:43:21]

Created Revert of Replace requirePageReconstruction with setCustomUserAgent

[commit-bot: I haz the power, 2017-04-17 12:43:34]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-04-17 12:54:45]

CQ is committing da patch.

Bot data: {"patchset_id": 1, "attempt_start_ts": 1492433000536900, "parent_rev":
"a86731ee4f39b32af514750aac3dffa44408268e", "commit_rev":
"57f76e7371706d96c3cd0915f8efc04fab8c1166"}

[commit-bot: I haz the power, 2017-04-17 12:55:02]

Description was changed from

==========
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/+/b647f8362016d8e5b7a614e92b72...

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
==========

to

==========
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/+/b647f8362016d8e5b7a614e92b72...

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}
Committed:
https://chromium.googlesource.com/chromium/src/+/57f76e7371706d96c3cd0915f8ef...
==========

[commit-bot: I haz the power, 2017-04-17 12:55:02]

Committed patchset #1 (id:1) as
https://chromium.googlesource.com/chromium/src/+/57f76e7371706d96c3cd0915f8ef...

[liaoyuke, 2017-04-17 19:52:07]

On 2017/04/17 12:55:02, commit-bot: I haz the power wrote:
> Committed patchset #1 (id:1) as
>
https://chromium.googlesource.com/chromium/src/+/57f76e7371706d96c3cd0915f8ef...

lgtm.