MediaQueryEvaluator should use default style for evaluating "rem".

LayoutTests/fast/frames/seamless/media-query-basing-rem-unit-expected.txt

Index: LayoutTests/fast/frames/seamless/media-query-basing-rem-unit-expected.txt
diff --git a/LayoutTests/fast/frames/seamless/media-query-basing-rem-unit-expected.txt b/LayoutTests/fast/frames/seamless/media-query-basing-rem-unit-expected.txt
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+Test for issue 258200:CSS Media Queries are basing "rem" unit on style data.
+
+On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
+
+
+PASS iframe.seamless is true
+PASS window.getComputedStyle(iframe.contentDocument.body).backgroundColor is "rgb(0, 128, 0)"
+PASS window.getComputedStyle(iframe.contentDocument.body).backgroundColor is "rgb(0, 128, 0)"
+PASS successfullyParsed is true
+
+TEST COMPLETE
+Resize the window to be larger than #1, but smaller than #2. If the screen turns red, the browser is basing 'rem' units in MQs on style data, which is a spec violation. (It should be equal to the initial font size, same as the 'em' unit, to avoid circularity issues.)
+