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

Review URL: https://codereview.chromium.org/669883003

git-svn-id: svn://svn.chromium.org/blink/trunk@184878 bbb929c8-8fbe-4397-9dbb-9b2b20218538