Create blink_headers target for header-only dependencies

Some code in Chromium wants to use Blink's public API headers for the definitions
of types, enums, structs etc without actually linking against the Blink library
itself. This target sets up the build configuration so that relative include paths
in the Blink headers resolve correctly. Once Chromium targets are updated to reflect
their dependencies more accurately, we can get rid of the relative includes in
the public Blink API.

BUG=350097

Review URL: https://codereview.chromium.org/189393003

git-svn-id: svn://svn.chromium.org/blink/trunk@168709 bbb929c8-8fbe-4397-9dbb-9b2b20218538