Move Image related tests from web/tests/ to platform/

Move BitmapImageTest.cpp, GIFImageDecoderTest.cpp and JPEGImageDecoderTest.cpp.

BUG=353585

Review URL: https://codereview.chromium.org/203543002

git-svn-id: svn://svn.chromium.org/blink/trunk@169544 bbb929c8-8fbe-4397-9dbb-9b2b20218538