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This adds a new target which provides test support for Source/platform/.
These can be used in platform unit tests, or unit tests of targets which
depend on platform.

GYP: blink_platform_test_support (static_library)
GN: //third_party/WebKit/Source/platform:test_support (source_set)

Existing GTest print support for Source/platform/geometry/ types has been
made more consistent, and moved into this target. Both
blink_platform_unittests and webkit_unit_tests depend on it.

To avoid ODR violations, the declarations of these are in the headers
of the types that are being printed. If these are called outside unit tests,
a link error will occur.

This is similar to how ui/gfx/ does things in Chromium; it's seems like
a more reasonable approach than what we do today.

BUG=522198

Review URL: https://codereview.chromium.org/1285413007

git-svn-id: svn://svn.chromium.org/blink/trunk@200933 bbb929c8-8fbe-4397-9dbb-9b2b20218538