Allow middle-click scrolling to work for PDFs and print preview
Allow middle-click scrolling to work for PDFs and print preview

This tells the plugin not to handle middle-click events so that they are
properly handled by the container page and allows middle-click scrolling
to work.

BUG=463399

Review URL: https://codereview.chromium.org/976123002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#319366}

Chilledheart/chromium,PeterWangIntel/chromium-crosswalk,fujunwei/chromium-crosswalk,fujunwei/chromium-crosswalk,Chilledheart/chromium,Just-D/chromium-1,Fireblend/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,chuan9/chromium-crosswalk,fujunwei/chromium-crosswalk,Chilledheart/chromium,Just-D/chromium-1,Chilledheart/chromium,PeterWangIntel/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,fujunwei/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,Pluto-tv/chromium-crosswalk,ltilve/chromium,axinging/chromium-crosswalk,ltilve/chromium,Just-D/chromium-1,chuan9/chromium-crosswalk,Fireblend/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Just-D/chromium-1,hgl888/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,fujunwei/chromium-crosswalk,Chilledheart/chromium,hgl888/chromium-crosswalk,Chilledheart/chromium,chuan9/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,Fireblend/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Fireblend/chromium-crosswalk,Pluto-tv/chromium-crosswalk,chuan9/chromium-crosswalk,axinging/chromium-crosswalk,fujunwei/chromium-crosswalk,Just-D/chromium-1,hgl888/chromium-crosswalk,Fireblend/chromium-crosswalk,Pluto-tv/chromium-crosswalk,ltilve/chromium,Just-D/chromium-1,axinging/chromium-crosswalk,ltilve/chromium,hgl888/chromium-crosswalk,Fireblend/chromium-crosswalk,axinging/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Just-D/chromium-1,Just-D/chromium-1,PeterWangIntel/chromium-crosswalk,chuan9/chromium-crosswalk,Just-D/chromium-1,chuan9/chromium-crosswalk,Fireblend/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,axinging/chromium-crosswalk,chuan9/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Chilledheart/chromium,PeterWangIntel/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,ltilve/chromium,fujunwei/chromium-crosswalk,hgl888/chromium-crosswalk,Fireblend/chromium-crosswalk,hgl888/chromium-crosswalk,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Fireblend/chromium-crosswalk,axinging/chromium-crosswalk,ltilve/chromium,axinging/chromium-crosswalk,chuan9/chromium-crosswalk,fujunwei/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Chilledheart/chromium,fujunwei/chromium-crosswalk,ltilve/chromium,ltilve/chromium,hgl888/chromium-crosswalk,hgl888/chromium-crosswalk,axinging/chromium-crosswalk,chuan9/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Chilledheart/chromium,Pluto-tv/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,hgl888/chromium-crosswalk,ltilve/chromium,TheTypoMaster/chromium-crosswalk