Hidden windows should not keep Chrome alive.

For each app window, there is a ChromeAppDelegate which
holds a ScopedKeepAlive. This CL resets the ScopedKeepAlive
while the app window is hidden.

BUG=450710

Review URL: https://codereview.chromium.org/875273003

Cr-Commit-Position: refs/heads/master@{#316413}
(cherry picked from commit 39d75d5b5f11525e384745fd313d1d77ed7d0a88)

TBR=benwells@chromium.org

Review URL: https://codereview.chromium.org/927203002

Cr-Commit-Position: refs/branch-heads/2272@{#304}
Cr-Branched-From: 827a380cfdb31aa54c8d56e63ce2c3fd8c3ba4d4-refs/heads/master@{#310958}

• chrome/browser/apps/app_window_interactive_uitest.cc[diff]
• chrome/browser/ui/apps/chrome_app_delegate.cc[diff]
• chrome/browser/ui/apps/chrome_app_delegate.h[diff]
• chrome/test/data/extensions/platform_apps/hidden/test.js[diff]
• chrome/test/data/extensions/platform_apps/hidden_then_shown/empty.html[Added - diff]
• chrome/test/data/extensions/platform_apps/hidden_then_shown/manifest.json[Added - diff]
• chrome/test/data/extensions/platform_apps/hidden_then_shown/test.js[Added - diff]
• extensions/browser/app_window/app_delegate.h[diff]
• extensions/browser/app_window/app_window.cc[diff]
• extensions/shell/browser/shell_app_delegate.h[diff]