Revert of Prevent inherited style from propagating into a plugin placeholder. (patchset #3 id:40001 of https://codereview.chromium.org/669883003/)

Reason for revert:
Causing layout test fast/plugins/plugin-placeholder-inherit.html to fail.

Original issue's description:
> Prevent inherited style from propagating into a plugin placeholder.
> 
> Without setting 'all: initial', it's possible for text colours and such to be
> inherited by the placeholder, which is unexpected.
> 
> TEST=fast/plugins/plugin-placeholder-inherit.html
> BUG=364716
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=184878

TBR=esprehn@chromium.org,mkwst@chromium.org,jbroman@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=364716

Review URL: https://codereview.chromium.org/706623003

git-svn-id: svn://svn.chromium.org/blink/trunk@184880 bbb929c8-8fbe-4397-9dbb-9b2b20218538