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Crash occurs because we are referencing a member variable left_button_down_on_caption_ after the DefWindowProc call for WM_NCLBUTTONDOWN, which could destroy the window and in
turn the HWNDMessageHandler instance.

Fix is to reset the left_button_down_on_caption_ member before the DefWindowProc call.

BUG=581341
TEST=Covered by views_unittest WidgetTest.DestroyInSysCommandNCLButtonDownOnCaption

Review URL: https://codereview.chromium.org/1636183003

Cr-Commit-Position: refs/heads/master@{#371892}