Revert of Remove applyAuthorStyles flag from ShadowRoot. (https://codereview.chromium.org/143993006)

It turned out that Quickoffice doesn't have workarounds because shadow
combinators, which were expected to be workarounds, are available only when the
Experimental Web Platform Features flag turned on,

We should put applyAuthorStyles back for M34 temporary.  We'll drop
applyAuthorStyles after we have shadow combinators enabled in default.  I assume these
things will happen in M35.

This revert patch is also adding deprecation warning messages to the usage of
ShadowRoot.applyAuthorStyles.

BUG=342622

Review URL: https://codereview.chromium.org/162563002

git-svn-id: svn://svn.chromium.org/blink/trunk@167075 bbb929c8-8fbe-4397-9dbb-9b2b20218538