Make the allocator_features gyp target target-only

allocator_features uses buildflag_header to generate a header at
build time. In host/target builds, both the #host and #target
targets have a ninja rule for the same path, which is bad.
This CL makes allocator_features follow the same pattern of
base_debugging_flags (target-only, everything else explicitly refers
to the #target veriant)

I verified that the problem reproduces without this patch
by doing:
$ build/gyp_chromium -DOS=android -Duse_experimental_allocator_shim=1
$ ninja -w dupbuild=err -n -C out_android/Release/ all
ninja: warning: multiple rules generate gen/base/allocator/features.h. builds involving this target will not be correct;

And this CL fixes it.

BUG=593695

Review URL: https://codereview.chromium.org/1794943006

Cr-Commit-Position: refs/heads/master@{#381289}

base/allocator/allocator.gyp

@@ -380,9 +380,10 @@
     },  # 'allocator' target.
     {
       # GN: //base/allocator:features
+      # When referenced from a target that might be compiled in the host
+      # toolchain, always refer to 'allocator_features#target'.
       'target_name': 'allocator_features',
       'includes': [ '../../build/buildflag_header.gypi' ],
-      'toolsets': ['host', 'target'],
       'variables': {
         'buildflag_header_path': 'base/allocator/features.h',
         'buildflag_flags': [

base/base.gyp

@@ -22,7 +22,7 @@
       },
       'dependencies': [
         'allocator/allocator.gyp:allocator',
-        'allocator/allocator.gyp:allocator_features',
+        'allocator/allocator.gyp:allocator_features#target',
         'base_debugging_flags#target',
         'base_static',
         'base_build_date#target',