Add chrome_split_dll gyp variable, and duplicate chrome_main_dll target (reland)

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index 3a6badfe22204a2c4e4d7e68f3da3235c7c40930..a44285cf234af29cbb37131554ebd92ce00c5d19 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -888,6 +888,10 @@
     # to get incremental linking to be faster in debug builds.
     'incremental_chrome_dll%': '0',
 
+    # Experimental setting to break chrome.dll in to chrome_browser.dll and
+    # chrome_child.dll.
+    'chrome_split_dll%': '0',
+
     # The default settings for third party code for treating
     # warnings-as-errors. Ideally, this would not be required, however there
     # is some third party code that takes a long time to fix/roll. So, this
@@ -1809,6 +1813,9 @@
           '<(DEPTH)/base/allocator/allocator.gyp:type_profiler',
         ],
       }],
+      ['chrome_split_dll', {
+        'defines': ['CHROME_SPLIT_DLL'],
+      }],
       ['OS=="linux" and clang==1 and host_arch=="ia32"', {
         # TODO(dmikurube): Remove -Wno-sentinel when Clang/LLVM is fixed.
         # See http://crbug.com/162818.