Reland of Enable allocator shim for Android (https://codereview.chromium.org/1875043003/)

base/allocator/README.md

 This document describes how malloc / new calls are routed in the various Chrome
 platforms.
 
 Bare in mind that the chromium codebase does not always just use `malloc()`.
 Some examples:
  - Large parts of the renderer (Blink) use two home-brewed allocators,
    PartitionAlloc and BlinkGC (Oilpan).
  - Some subsystems, such as the V8 JavaScript engine, handle memory management
    autonomously.
  - Various parts of the codebase use abstractions such as `SharedMemory` or
@@ skipping 79 matching lines @@
 On most platform, Chrome overrides the malloc / operator new symbols (and
 corresponding free / delete and other variants). This is to enforce security
 checks and lately to enable the
 [memory-infra heap profiler][url-memory-infra-heap-profiler].  
 Historically each platform had its special logic for defining the allocator
 symbols in different places of the codebase. The unified allocator shim is
 a project aimed to unify the symbol definition and allocator routing logic in
 a central place.
 
  - Full documentation: [Allocator shim design doc][url-allocator-shim].
- - Current state: Available and enabled by default on Linux and CrOS.
+ - Current state: Available and enabled by default on Linux, CrOS and Android.
  - Tracking bug: [https://crbug.com/550886][crbug.com/550886].
  - Build-time flag: `use_experimental_allocator_shim`.
 
 **Overview of the unified allocator shim**  
 The allocator shim consists of three stages:
 ```
 +-------------------------+    +-----------------------+    +----------------+
 |     malloc & friends    | -> |       shim layer      | -> |   Routing to   |
 |    symbols definition   |    |     implementation    |    |    allocator   |
 +-------------------------+    +-----------------------+    +----------------+
@@ skipping 76 matching lines @@
 Related links
 -------------
 - [Unified allocator shim doc - Feb 2016][url-allocator-shim]
 - [Allocator cleanup doc - Jan 2016][url-allocator-cleanup]
 - [Proposal to use PartitionAlloc as default allocator](https://crbug.com/339604)
 - [Memory-Infra: Tools to profile memory usage in Chrome](components/tracing/docs/memory_infra.md)
 
 [url-allocator-cleanup]: https://docs.google.com/document/d/1V77Kgp_4tfaaWPEZVxNevoD02wXiatnAv7Ssgr0hmjg/edit?usp=sharing
 [url-memory-infra-heap-profiler]: components/tracing/docs/heap_profiler.md
 [url-allocator-shim]: https://docs.google.com/document/d/1yKlO1AO4XjpDad9rjcBOI15EKdAGsuGO_IeZy0g0kxo/edit?usp=sharing