Disable tcmalloc when building for valgrind

tcmalloc doesn't seem to play well with Valgrind. See crbug.com/582398
and the discussion in crrev.com/1642383002 .
On top of that, routing to the system allocator seems consistent with
what we already do in the case of other tools. Today we already set
use_allocator='none' if (asan==1 or msan==1 or lsan==1 or tsan==1).

BUG=582398
TEST=build with GYP_DEFINES="build_for_tool=memcheck clang=1 profiling=1" and run with valgrind out/Release/chrome --no-sandbox

Review URL: https://codereview.chromium.org/1668643002

Cr-Commit-Position: refs/heads/master@{#373835}

build/common.gypi

@@ -2292,6 +2292,10 @@
       # TODO(rnk): Kill off variables that no one else uses and just implement
       # them under a build_for_tool== condition.
       ['build_for_tool=="memcheck" or build_for_tool=="tsan"', {
+
+        # tcmalloc causes Valgrind failures. Discussion in crrev.com/1642383002.
+        'use_allocator%': 'none',
+
         # gcc flags
         'mac_debug_optimization': '1',
         'mac_release_optimization': '1',