Fix flaky NativeWidgetMacTest.SetCursor test.
Fix flaky NativeWidgetMacTest.SetCursor test.

This CL fixes the flaky NativeWidgetMacTest.SetCursor test. This test sometimes
fails on EXPECT_FALSE([NSCursor currentCursor]).

The reason for this flaky behaviour is that all NativeWidgetMacTests share the
same NSApp instance. SetCursor begins with a sanity check that the cursor stack
is empty, but that's not true if other tests have started manipulating the
shared NSApp instance. In this case, the call to [NSApp hide:nil] in
NativeWidgetMacTest.HideAndShowExternally leads to the setting of a cursor for
the application. Hence whenever NativeWidgetMacTest.SetCursor ran after
NativeWidgetMacTest.HideAndShowExternally, it failed.

BUG=521460

Review URL: https://codereview.chromium.org/1810283002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#382250}

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