GN: blink_platform test targets

Adds the blink_heap_unittests and blink_platform_unittests target, bringing
Source/platform/BUILD.gn to parity with Source/platform/blink_platform_tests.gyp.
These fail to link on mac with missing symbols in libjpeg_turbo due to a missing
yasm target but should otherwise be functional.

Review URL: https://codereview.chromium.org/317583004

git-svn-id: svn://svn.chromium.org/blink/trunk@175447 bbb929c8-8fbe-4397-9dbb-9b2b20218538