GN: Enable blink platform heap targets, put all tests behind flag

This puts the blink heap asm targets in the GN build set and enables
both blink platform test targets behind a single flag. They compile, link
and run successfully on mac but (probably) don't link yet on linux due
to ffmpeg deps.

R=brettw@chromium.org

Review URL: https://codereview.chromium.org/332123002

git-svn-id: svn://svn.chromium.org/blink/trunk@176174 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Removing using declarations that import names in the C++ Standard library.

The original CL https://codereview.chromium.org/329183002/ got reverted in
https://codereview.chromium.org/330933002/ as it was causing failures on the
Chromium Linux Tsan v2 bot.

The problem was that std::abs(int) is found in <cstdlib> but we apparently only
include <cmath> everywhere. So the compiler only knows about std::abs(float) and
variants.

There were two ways of fixing this:
1. include <cstdlib> whereever std::abs(int) is used.
2. use abs() as it was done earlier.

Because we already include <cmath> everywhere, and Since C++11, additional
overloads are provided in the header <cmath> for the integral types as well, so
its better to directly just use abs() rather than including an extra header file
This CL uses the second way.

BUG=None

Review URL: https://codereview.chromium.org/333173002

git-svn-id: svn://svn.chromium.org/blink/trunk@176179 bbb929c8-8fbe-4397-9dbb-9b2b20218538