Enable hi-dpi support on Linux.

build/common.gypi

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # IMPORTANT:
 # Please don't directly include this file if you are building via gyp_chromium,
 # since gyp_chromium is automatically forcing its inclusion.
 {
   # Variables expected to be overriden on the GYP command line (-D) or by
   # ~/.gyp/include.gypi.
@@ skipping 192 matching lines @@
             'toolkit_views%': 0,
           }],
 
           # Embedded builds use aura without ash or views.
           ['embedded==1', {
             'use_aura%': 1,
             'use_ash%': 0,
             '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,
           }],
 
           # Enable the OpenSSL backend on Mac OS and Windows.
           ['OS=="mac" or OS=="win"', {
             'use_openssl%': 1,
           }],
 
           # Enable App Launcher everywhere but mobile.
           ['OS!="ios" and OS!="android"', {
@@ skipping 5802 matching lines @@
     # settings in target dicts.  SYMROOT is a special case, because many other
     # Xcode variables depend on it, including variables such as
     # PROJECT_DERIVED_FILE_DIR.  When a source group corresponding to something
     # like PROJECT_DERIVED_FILE_DIR is added to a project, in order for the
     # files to appear (when present) in the UI as actual files and not red
     # red "missing file" proxies, the correct path to PROJECT_DERIVED_FILE_DIR,
     # and therefore SYMROOT, needs to be set at the project level.
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }