Move remained platform tests from web/tests/ to platform/

Move GraphicsLayerTest, ImageLayerChromiumTest,
OpaqueRectTrackingContentLayerDelegateTest, FilterOperationsTest,
ImageFilterBuilderTest, and WEBPImageDecoderTest.

BUG=353585

Review URL: https://codereview.chromium.org/216423011

git-svn-id: svn://svn.chromium.org/blink/trunk@170681 bbb929c8-8fbe-4397-9dbb-9b2b20218538