[M108 Aura Migration][NaCl][PPFW] Fix memory leaks when reloading iframe

[M108 Aura Migration][NaCl][PPFW] Fix memory leaks when reloading iframe

Leaks of PepperHelper class instances.

Class PepperHelper is created in
ContentRendererClientEfl::RenderFrameCreated
it should be deleted together with render frame for which was created.
Removal of RenderFrame is signalized by call to OnDestruct method.
In case of PepperHelper this method is empty.

This patch adds proper implementation of this method to avoid leak.

from 94 Aura Patch:
https://review.tizen.org/gerrit/#/c/platform/framework/web/chromium-efl/+/280196/

Change-Id: I7aeaec172acab137e1e5d193a087de0248edaf23
Signed-off-by: pengxia <pengxia.shen@samsung.com>