Created placeholders for MallocHooks implementation and added define DART_USE_TCMALLOC to specifiy …

runtime/vm/vm.gypi

Index: runtime/vm/vm.gypi
diff --git a/runtime/vm/vm.gypi b/runtime/vm/vm.gypi
index a8c0e74ac4b04cbf916dea3ac8929de966de5729..becf025f69a177e3b56f4f43a582956678611712 100644
--- a/runtime/vm/vm.gypi
+++ b/runtime/vm/vm.gypi
@@ -61,6 +61,14 @@
             ],
           },
         }],
+	['OS == "linux" and asan == 0 and msan == 0 and tsan == 0', {

[bkonyi, 2017/01/05 23:29:04]

There was already a rule for 'OS == "linux' so I added a separate condition for
the tcmalloc dependency inclusion. Let me know if this isn't right. Also, I'll
fix the indentation during my next patch.

+          'dependencies': [
+            '../third_party/tcmalloc/tcmalloc.gypi:tcmalloc',

[zra, 2017/01/06 16:13:17]

I think it's not necessary to add the dependence on tcmalloc here. Instead maybe
just add a comment that the condition should be kept in sync with where we're
already adding the dependence. Normally we'd figure out how to add a variable
somewhere to keep them from going out of sync, but since gyp is going away soon
it's probably not worth spending time out.

[bkonyi, 2017/01/06 20:23:34]

Done.

+          ],
+          'defines': [
+            'DART_USE_TCMALLOC'
+          ],
+        }],
         ['OS=="android" and _toolset=="host"', {
           'link_settings': {
             'libraries': [
@@ -105,6 +113,14 @@
             ],
           },
         }],
+        ['OS == "linux" and asan == 0 and msan == 0 and tsan == 0', {
+          'dependencies': [

[zra, 2017/01/06 16:13:17]

ditto

[bkonyi, 2017/01/06 20:23:34]

Done.

+            '../third_party/tcmalloc/tcmalloc.gypi:tcmalloc',
+          ],
+          'defines': [
+            'DART_USE_TCMALLOC'
+          ],
+        }],
         ['OS=="android" and _toolset=="host"', {
           'link_settings': {
             'libraries': [
@@ -149,6 +165,14 @@
             ],
           },
         }],
+        ['OS == "linux" and asan == 0 and msan == 0 and tsan == 0', {
+          'dependencies': [

[zra, 2017/01/06 16:13:17]

ditto

[bkonyi, 2017/01/06 20:23:34]

Done.

+            '../third_party/tcmalloc/tcmalloc.gypi:tcmalloc',
+          ],
+          'defines': [
+            'DART_USE_TCMALLOC'
+          ],
+        }],
         ['OS=="android" and _toolset=="host"', {
           'link_settings': {
             'libraries': [
@@ -194,6 +218,14 @@
             ],
           },
         }],
+        ['OS == "linux" and asan == 0 and msan == 0 and tsan == 0', {
+          'dependencies': [

[zra, 2017/01/06 16:13:17]

ditto

[bkonyi, 2017/01/06 20:23:34]

Done.

+            '../third_party/tcmalloc/tcmalloc.gypi:tcmalloc',
+          ],
+          'defines': [
+            'DART_USE_TCMALLOC'
+          ],
+        }],
         ['OS=="android" and _toolset=="host"', {
           'link_settings': {
             'libraries': [