test: Fix test failure with Chromium-based Edge
test: Fix test failure with Chromium-based Edge

One particular test used a direct UA string check to detect Edge, but
did so differently than shaka.util.Platform does in isEdge().  So the
test did not detect Chromium-based Edge correctly.  This fixes the
test by using shaka.util.Platform.isEdge().

Change-Id: I1eb23776f4c2c4ef531b20d34b4d56c931175e6e

shaka-project/shaka-player,tvoli/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,tvoli/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player,tvoli/shaka-player