Bring ewk_context_memory_sampler_* back.

Bring ewk_context_memory_sampler_* back.

These APIs were wrongly removed in [1]. In order to build
t-mobile-browser app out of the box for chromium-efl/beta,
chromium-efl/dev and webkit-efl, this patch brings it back.

They are only stub APIs.

Patch reverts [1] partially.

[1] http://165.213.202.130:8080/#/c/71214/

Bug: http://107.108.218.239/bugzilla/show_bug.cgi?id=8497
Reviewed by: Kamil Klimek, SeungSeop Park

Change-Id: I7674f8d40187db95ecca25a2ea5cf4805c922235
Signed-off-by: Antonio Gomes <a1.gomes@samsung.com>