WebProcessMemoryDump and WebMemoryAllocatorDump are abstract interfaces
because they lived in public/platform. Now they live in platform so there
is no reason to have such abstractions. Unify them with their impl classes.
This CL also replaces WebString with String.

BUG=605822

Review-Url: https://codereview.chromium.org/2028483002
Cr-Commit-Position: refs/heads/master@{#396803}