allocator: remove ENABLE_MEMORY_TASK_PROFILER and use only USE_ALLOCATOR_SHIM

After the recent cleanup of crrev.com/2907563002, the flag
ENABLE_MEMORY_TASK_PROFILER became redundant and now is just
matching 1:1 USE_ALLOCATOR_SHIM. Use that instead.
No intended behavior change.
Also fix mistake of using defined(X) instead
of BUILDFLAG(X).

BUG=550886

Change-Id: I03a840b22861182208f1297a0c3f1152fd985c32
Reviewed-on: https://chromium-review.googlesource.com/520768
Commit-Queue: Primiano Tucci <primiano@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Sigurður Ásgeirsson <siggi@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#476364}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 05f56d412142ccc4f2d3d1b0defa2a05ee9a25bc
diff --git a/tools/gn/bootstrap/bootstrap.py b/tools/gn/bootstrap/bootstrap.py
index 0b03d26..e806761 100755
--- a/tools/gn/bootstrap/bootstrap.py
+++ b/tools/gn/bootstrap/bootstrap.py
@@ -172,7 +172,6 @@
   write_buildflag_header_manually(root_gen_dir, 'base/debug/debugging_flags.h',
       {
           'ENABLE_PROFILING': 'false',
-          'ENABLE_MEMORY_TASK_PROFILER': 'false',
           'CAN_UNWIND_WITH_FRAME_POINTERS': 'false'
       })