Linux: add gyp flag for enabling tcmalloc.

base/base.gyp

Index: base/base.gyp
diff --git a/base/base.gyp b/base/base.gyp
index 7be1225eb2ccfa02545573b8fb0badeeb1d8dbe9..bf6d9d2464a4b817602b1fba277e4accf9101947 100644
--- a/base/base.gyp
+++ b/base/base.gyp
@@ -365,6 +365,9 @@
       ],
       'conditions': [
         [ 'OS == "linux"', {
+            'variables' : {
+              'linux_use_tcmalloc%': 0,
+            },
             'sources/': [ ['exclude', '_(mac|win|chromeos)\\.cc$'],
                           ['exclude', '\\.mm?$' ] ],
             'sources!': [
@@ -378,6 +381,12 @@
                   'sources/': [ ['include', '_chromeos\\.cc$'] ]
                 },
               ],
+              [ 'linux_use_tcmalloc==1', {
+                  'dependencies': [
+                    '../third_party/tcmalloc/tcmalloc.gyp:tcmalloc',
+                  ],
+                },
+              ],
             ],
             'dependencies': [
               '../build/util/build_util.gyp:lastchange',