Use a qualified path to blink resources, blink part.

This is technically a prerequisite to https://codereview.chromium.org/430083002
(In practice, content/ probably already has an include path for
SHARED_INTERMEDIATE_DIR.)

The gn file didn't set the output directory correctly, so that part
is a hard prerequisite.

BUG=400860
TBR=eseidel

Review URL: https://codereview.chromium.org/431883002

git-svn-id: svn://svn.chromium.org/blink/trunk@179670 bbb929c8-8fbe-4397-9dbb-9b2b20218538