Reland [blink-in-js] Migrate resources required for blink-in-js to grd - part 3

The earlier commit for this was reverted here https://codereview.chromium.org/557543004
as it broke the compilation on Linux GN (dbg) which is tree closer.
http://build.chromium.org/p/chromium.linux/builders/Linux%20GN%20(dbg)

Adding the changes required, the flag --for-testing, in the Source/core/BUILD.gn to include
this flag.

BUG=312586

Review URL: https://codereview.chromium.org/569063002

git-svn-id: svn://svn.chromium.org/blink/trunk@181971 bbb929c8-8fbe-4397-9dbb-9b2b20218538