Move GraphicsContextTest to platform/ and fix the test environment.

Previously, no Blink platform was initialized, leading to a segfault
when a trace event occurred in NativeImageSkia::draw.

This was fixed by making the platform unit test runner provide a
minimal platform by default. The gyp target was adjusted to match the
way blink_heap_unittests, for example, sets up its environment.

This seems like a reasonable compromise between having a null platform
and having a more complete platform implemented on the Chromium side,
like the web unit tests have.

BUG=353585

Review URL: https://codereview.chromium.org/206473009

git-svn-id: svn://svn.chromium.org/blink/trunk@169982 bbb929c8-8fbe-4397-9dbb-9b2b20218538