Fixed the test WidgetTest.SynthesizeMouseMoveEvent
Fixed the test WidgetTest.SynthesizeMouseMoveEvent

The change:
Using ui::test::EventGenerator to emit a mouse event in the test;
right positioning the widget.
Doing that same way as in neighbour test cases.

The reason:
The WidgetTest.SynthesizeMouseMoveEvent was failing on MacOSX, because
of two reasons.
First, initially emitted mouse event was handled wrong way, entered
views::ViewTargeter::FindTargetForEvent() and met NOTREACHED.
Second, the widget's rect was random and too tiny on MacOSX (size 1*26),
so synthesized event (5,5) was rejected as missing the rectangle.
Both problems are specific to MacOSX implementation of views and do not
appear on Windows.

BUG=518056

TEST=run views_unittests \
         --gtest_filter=WidgetTest.SynthesizeMouseMoveEvent
on MacOSX (tested on 10.10.5 Yosemite with retina display).

Review URL: https://codereview.chromium.org/1431293002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#359846}

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