Revert "Add web::NavigationContext::IsPlaceholderNavigation"

This reverts commit 7b0f2bb852e0280fb3ce78860f7691e4e64ae6f3.

Reason for revert: Fails multiple IOS builds including Builder ios-slimnav <https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/ios-slimnav/191>

Link to sample failure logs : https://logs.chromium.org/logs/chromium/buildbucket/cr-buildbucket.appspot.com/8929398908718170816/+/steps/ios_chrome_unittests__iPhone_6s_iOS_12.1_/0/logs/TextToSpeechListenerTest.ValidAudioDataTest/0


Original change's description:
> Add web::NavigationContext::IsPlaceholderNavigation
> 
> This is a step in multistep refactoring. The next steps will be:
> 
> 1.) web::NavigationContext::URL is never set to placeholder
>     URL and always represents navigation URL
> 2.) Same web::NavigationContext will be reused for placeholder
>     navigation to extend it's lifetime.
> 3.) WebStateObserver::DidFinishNavigation will be caller after
>     placeholder navigation is finished and will use original
>     web::NavigationContext passed to WebStateObserver::DidStartNavigation
> 
> This will partially fix crbug.com/903497 and will call
> WebStateObserver::DidFinishNavigation after committed URL actually
> changed.
> 
> Bug: 903497
> Change-Id: Ib6ba96d664ac352d038787aa74ac844d07282789
> Reviewed-on: https://chromium-review.googlesource.com/c/1331044
> Reviewed-by: Danyao Wang <danyao@chromium.org>
> Commit-Queue: Eugene But <eugenebut@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#609398}

TBR=eugenebut@chromium.org,danyao@chromium.org

Change-Id: I8d7c9b66d752a792761d95ccf348f045eaa9de03
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 903497
Reviewed-on: https://chromium-review.googlesource.com/c/1343351
Reviewed-by: Jun Choi <hongjunchoi@chromium.org>
Commit-Queue: Jun Choi <hongjunchoi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609494}
10 files changed