Enable hi-dpi support on Linux.

With the recent bugfixes (see BUG=), hi-dpi support on Linux works well enough
to enable it by default.

BUG=143619

Review URL: https://codereview.chromium.org/1028703004

Cr-Commit-Position: refs/heads/master@{#321865}

build/common.gypi

@@ -210,8 +210,8 @@
             'toolkit_views%': 0,
           }],
 
-          # Enable HiDPI on Mac OS, Chrome OS and Windows.
-          ['OS=="mac" or chromeos==1 or OS=="win"', {
+          # Enable HiDPI on Mac OS, Chrome OS, Windows and Linux.
+          ['OS=="mac" or chromeos==1 or OS=="win" or OS=="linux"', {
             'enable_hidpi%': 1,
           }],
 

build/config/ui.gni

@@ -62,4 +62,4 @@ use_ozone_evdev = use_ozone
 
 use_clipboard_aurax11 = is_linux && use_aura && use_x11
 
-enable_hidpi = is_mac || is_chromeos || is_win
+enable_hidpi = is_mac || is_chromeos || is_win || is_linux