Restore -Wall to Linux build and set up for -Werror.

* Add -Wall to build/common.gypi (and -Werror, commented out for now).
* Have build/external_code.gypi remove -Wall (and -Werror).
* Remove chromium_code definition from build/all.gyp.
* Remove chromium_code definitions from third_party/ *.gyp files.
* Remove scons-specific -Werror removal in webkit.gyp.
* Remove unused variables from:
  base/clipboard_linux.cc
  chrome/browser/gtk/download_shelf_gtk.cc
  chrome/browser/gtk/bookmark_bar_gtk.cc
Review URL: http://codereview.chromium.org/66001

git-svn-id: http://src.chromium.org/svn/trunk/src/build@13478 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

all.gyp

@@ -4,7 +4,6 @@
 
 {
   'variables': {
-    'chromium_code': 1,
     'conditions': [
       ['OS!="win"', {
         'all_gyps%': 1,

common.gypi

@@ -169,6 +169,9 @@
           '-fno-exceptions',
           '-msse2',
           '-mfpmath=sse',
+          '-Wall',
+          # TODO(sgk): when chromium_code targets are clean of warnings.
+          #'-Werror',
         ],
         'ldflags': [
           '-m32',

external_code.gypi

@@ -4,6 +4,14 @@
 
 {
   'conditions': [
+    [ 'OS=="linux"', {
+      'target_defaults': {
+        'cflags!': [
+          '-Wall',
+          '-Werror',
+        ],
+      },
+    }],
     [ 'OS=="win"', {
       'target_defaults': {
         'defines': [