Make it possible to disable udev in the content API on linux

This is useful for embedded linux setups, which often don't include udev support.

TEST=Build content_shell on linux with use_udev=0 then run ldd on output binaries to verify libudev is not listed (and is listed if built with use_udev=1 or unspecified)

BUG=318315,318413

Review URL: https://codereview.chromium.org/12374068

git-svn-id: http://src.chromium.org/svn/trunk/src/build@236204 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

common.gypi

@@ -506,6 +506,13 @@
           'use_nss%': 0,
         }],
 
+        # libudev usage.  This currently only affects the content layer.
+        ['OS=="linux"', {
+          'use_udev%': 1,
+        }, {
+          'use_udev%': 0,
+        }],
+
         # Flags to use X11 on non-Mac POSIX platforms.
         ['OS=="win" or OS=="mac" or OS=="ios" or OS=="android" or use_ozone==1', {
           'use_x11%': 0,
@@ -850,6 +857,7 @@
     'use_cras%': '<(use_cras)',
     'use_openssl%': '<(use_openssl)',
     'use_nss%': '<(use_nss)',
+    'use_udev%': '<(use_udev)',
     'os_bsd%': '<(os_bsd)',
     'os_posix%': '<(os_posix)',
     'use_dbus%': '<(use_dbus)',
@@ -2146,6 +2154,9 @@
       ['native_memory_pressure_signals==1', {
         'defines': ['SYSTEM_NATIVELY_SIGNALS_MEMORY_PRESSURE'],
       }],
+      ['use_udev==1', {
+        'defines': ['USE_UDEV'],
+      }],
       ['fastbuild!=0', {
         'xcode_settings': {
           'GCC_GENERATE_DEBUGGING_SYMBOLS': 'NO',