Create blink_headers target for header-only dependencies

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
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=168709

[abarth-chromium, 2014-03-07 01:29:32]

lgtm

[jamesr, 2014-03-07 01:30:28]

The CQ bit was checked by jamesr@chromium.org

[commit-bot: I haz the power, 2014-03-07 01:30:36]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/jamesr@chromium.org/189393003/1

[commit-bot: I haz the power, 2014-03-07 02:30:58]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-03-07 02:30:58]

Try jobs failed on following builders: win_blink_rel

[tkent, 2014-03-07 05:27:47]

The CQ bit was checked by tkent@chromium.org

[commit-bot: I haz the power, 2014-03-07 05:27:54]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/jamesr@chromium.org/189393003/1

[commit-bot: I haz the power, 2014-03-07 06:47:52]

Change committed as 168709