XSLImportRule should not be ResourceClient to make XSLImportRule independent of blink.

Instead, it should use synchronous ResourceRequest to obtain imported xsl-stylesheet.

TEST=fast/xsl/transform-xhr-doc.xhtml
BUG=367689

Review URL: https://codereview.chromium.org/507583002

git-svn-id: svn://svn.chromium.org/blink/trunk@181536 bbb929c8-8fbe-4397-9dbb-9b2b20218538