Auto merge of #1610 - glennw:wrench-windows-build-break, r=glennw
Auto merge of #1610 - glennw:wrench-windows-build-break, r=glennw

Add the mut keyword for font_key_from_native_handle() in windows plat…

…form.

Fix the build break in windows. The add_native_font() call in this
function needs a mutable variable.

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