Move GraphicsContextTest to platform/ and fix the test environment.

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
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=169982

[jbroman, 2014-03-23 15:03:19]

I noticed that the only reason GraphicsContextTest doesn't run properly in
blink_platform_unittests right now is that it calls NativeImageSkia::draw, which
has a trace event in it (and that tried to access blink::Platform::current(),
which was null).

It seems to me that all platform tests can reasonably expect to have a non-null
platform, and to be able to use trace events without worrying about whether the
code can be called in test.

I've imitated the pattern of wtf_unittests and blink_heap_unittests to provide a
RunAllTests that does implement a minimal platform. Please take a look.

[dshwang, 2014-03-24 11:27:28]

This CL allows platform/ get the similar test harness to wtf/. non-owner lgtm.

[jamesr, 2014-03-25 21:17:07]

lgtm - thanks!

[jbroman, 2014-03-25 21:18:36]

The CQ bit was checked by jbroman@chromium.org

[commit-bot: I haz the power, 2014-03-25 21:18:47]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/jbroman@chromium.org/206473009/20001

[commit-bot: I haz the power, 2014-03-25 23:51:57]

Change committed as 169982