Move references to picker_resources genfiles from core.gyp to web.gyp

These files are only used by Source/web/, so there should be no need to
build them into core.

This patch fixes flaky build breakages that depended on the ordering of building
Source/web vs Source/core.

R=thakis@chromium.org
BUG=325001

Review URL: https://codereview.chromium.org/93933004

git-svn-id: svn://svn.chromium.org/blink/trunk@163108 bbb929c8-8fbe-4397-9dbb-9b2b20218538