Restructure chromium-efl.gyp (build entry point)

Restructure chromium-efl.gyp (build entry point)

Patch creates a "build_all"-like target for EWK,
where all the sample apps are set as 'dependencies'.
It also deliberately removes the explicit inclusion of
efl_integration.gypi (where the 'chromium-ewk' target is
declared).

Reasoning is that 'chromium-ewk' is set as an explicit
dependecy of all three targets currently listed in chromium-ewk.gyp,
so its inclusion is redundant.
Also, the explicit inclusion of efl_integration.gypi from
chromium-ewk.gyp caused GYP error when we try to set 'chromium-ewk'
target as a dependency for 'xwalk' target:

[XXs] IndexError: string index out of range while reading
includes of /(..)/tizen_src/ewk/chromium-ewk.gyp while trying to load
/(..)/tizen_src/ewk/chromium-ewk.gyp

Reference: https://groups.google.com/forum/#!topic/gyp-developer/J-0xR5wye30

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

Change-Id: Ic30be2a2dd218c7da3a1b5398b222b1c926ee941
Signed-off-by: Antonio Gomes <a1.gomes@samsung.com>