Revert of Plugins: Remove Shadow DOM Plugin Placeholder

Revert of Plugins: Remove Shadow DOM Plugin Placeholder (patchset #4 id:70001 of https://codereview.chromium.org/1314643002/ )

Reason for revert:
Caused several blink layout and unit tests to fail: https://build.chromium.org/p/chromium.webkit/builders/WebKit%20Linux/builds/53104

unit tests failures:
PluginPlaceholderImplTest.MessageIsShown
PluginPlaceholderImplTest.NotCloseable
PluginPlaceholderImplTest.MessageDoesNotRunScripts
FrameLoaderClientImplTest.CreatePluginPlaceholderForwardsToWebFrameClient
PluginPlaceholderImplTest.MessageDoesNotAcceptElements
PluginPlaceholderImplTest.Closeable

layout test unexpected_failures:
fast/plugins/plugin-placeholder-structured.html
fast/plugins/plugin-placeholder-close.html
fast/dom/shadow/remove-shadowroot-from-document-and-destroy-crash.html
fast/plugins/plugin-placeholder-inherit.html
fast/plugins/plugin-placeholder-csp.html
fast/plugins/plugin-placeholder-focus.html

Original issue's description:
> Plugins: Remove Shadow DOM Plugin Placeholder
>
> It's not under active development and doesn't support Plugin Power Saver. It's
> overall a good approach, but unfortunately Shadow DOM doesn't have a good
> mechanism to call into C++ at present.
>
> Plugins themselves will (hopefully) be obsolete by the time it does.
>
> BUG=524115
>
> Committed: https://crrev.com/0c85f0c129bd64e6f3e5aabb659621d8cf65c690
> Cr-Commit-Position: refs/heads/master@{#345496}

TBR=jbroman@chromium.org,sky@chromium.org,kinuko@chromium.org,tommycli@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=524115
Committed: https://crrev.com/95a85137cf4a9e74cce2f7ab90f46c2e57e41f62
Cr-Commit-Position: refs/heads/master@{#345524}

[dstockwell, 2015-08-26 01:34:49]

Created Revert of Plugins: Remove Shadow DOM Plugin Placeholder

[commit-bot: I haz the power, 2015-08-26 01:35:00]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1319543004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1319543004/1

[commit-bot: I haz the power, 2015-08-26 01:35:30]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-26 01:36:25]

Patchset 1 (id:??) landed as
https://crrev.com/95a85137cf4a9e74cce2f7ab90f46c2e57e41f62
Cr-Commit-Position: refs/heads/master@{#345524}