MacViews: Fix views_unittests CustomButtonTest.HoverStateOnVisibilityChange
MacViews: Fix views_unittests CustomButtonTest.HoverStateOnVisibilityChange

Tests added in r188803 and updated in r329132. Fails on Mac because the
position of the cursor, as captured by gfx::Screen::GetCursorScreenPoint(),
is actually returning the location of the user's mouse at the time the test
was run. This is fixed by using the EventGenerator, which simulates the
mouse cursor's location and behaviour for more consistent results.

BUG=579380

Review URL: https://codereview.chromium.org/1636293002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#375698}

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