[android] Make PartitionAlloc-Everywhere default on Android.

Switch to PartitionAlloc for malloc() on Android. According to dev
experiments, this is either neutral or a win for memory, performance and
stability, with the exception of median GPU process footprint, which
regresses by a small amount, but is being addressed.

Note to sheriffs: If something breaks in this range on Android, this CL
                  is a likely culprit.

Note to perf sheriffs: This is expected to move a lot of performance
                       metrics, in both directions.

For details, see linked bugs, and for performance data from experiments,
https://docs.google.com/document/d/1asZy8Dy0m97AECTTU2jTbgwc-mwI9BLOMVYKKiQAp04/edit
(Google internal).

Bug: 1121427, 998048
Change-Id: Ice8d5c65e814a3086d121f8e8641f874d6b9e68b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2625869
Commit-Queue: Benoit L <lizeb@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Bartek Nowierski <bartekn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843120}

base/allocator/allocator.gni

@@ -18,10 +18,10 @@ if ((is_linux || is_chromeos || is_android || is_apple ||
   _default_use_allocator_shim = false
 }
 
-if (_default_use_allocator_shim && is_win) {
+if (_default_use_allocator_shim && (is_win || is_android)) {
   _default_allocator = "partition"
-} else if (is_android || is_apple || _is_using_sanitizers || is_win ||
-           is_fuchsia || ((is_linux || is_chromeos) && target_cpu == "arm64") ||
+} else if (is_apple || _is_using_sanitizers || is_win || is_fuchsia ||
+           ((is_linux || is_chromeos) && target_cpu == "arm64") ||
            (is_cast_audio_only && target_cpu == "arm")) {
   # Temporarily disable tcmalloc on arm64 linux to get rid of compilation
   # errors.