Remove unneeded dependencies when building chromium-efl-libs and crosswalk-bin

Remove unneeded dependencies when building chromium-efl-libs and crosswalk-bin

Patch adds a 'building_strategy' variable to chromium-efl
GYP machinery. In build/system.gypi, it can be used to
filter dependencies in and out, according to what target
is being built: ewk, chromium-efl-libs or crosswalk-bin.

Patch could have replaced the existing 'building_crosswalk_bin'
by 'building_strategy', but the former is a crosswalk
upstream GYP variable, which has it own meaning in
xwalk/build/build.gypi.

Bug: http://107.108.218.239/bugzilla/show_bug.cgi?id=13316
Reviewed by: SeungSeop Park, arno renevier

Change-Id: I724341bed2ae1cbae1dc50bbc18e5a5451f35973
Signed-off-by: Antonio Gomes <a1.gomes@samsung.com>