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This reverts commit 401cd174.

Reason for revert: Changes in content/browser are not needed and may
cause existing other use cases of ShouldSuppressDialogs() to break.
The |delegate_| in content/browser could be a class different than prerender_contents. 

Original change's description:
> Disable logic to kill prerender when JavaScript alert fires
> 
> This is no longer needed with NoStatePrefetch
> 
> Change-Id: Iedfd77ae5d221609eadfdb001ff04fb3b83e100e
> Bug: 755936
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1978496
> Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
> Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
> Commit-Queue: Tarun Bansal <tbansal@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#727345}

TBR=kinuko@chromium.org,tbansal@chromium.org,ryansturm@chromium.org

Change-Id: I59533f852d55e0540730540b23f563efe3e752b8
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 755936
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1981330Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#727348}