[page_allocator] Restrict ASLR range on 64-bit ARM Android

- This is a speculative fix to address a large performance regression
  on Android WebView.
- Restricts the range on 64 bit ARM Android to 0x20000000 - 0x60000000.

Bug: chromium:837640
Change-Id: I913ed62a9d360ae96aeaab80f468c18f357f956f
Reviewed-on: https://chromium-review.googlesource.com/1062012Reviewed-by: Chris Palmer <palmer@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559255}

base/allocator/partition_allocator/address_space_randomization.h

@@ -88,11 +88,22 @@ constexpr uintptr_t AslrMask(uintptr_t bits) {
 
     #elif defined(ARCH_CPU_ARM64)
 
+      #if defined(OS_ANDROID)
+
+      // Restrict the address range on Android to avoid a large performance
+      // regression in single-process WebViews. See https://crbug.com/837640.
+      constexpr uintptr_t kASLRMask = AslrMask(30);
+      constexpr uintptr_t kASLROffset = AslrAddress(0x20000000ULL);
+
+      #else
+
       // ARM64 on Linux has 39-bit user space. Use 38 bits since kASLROffset
       // could cause a carry.
       constexpr uintptr_t kASLRMask = AslrMask(38);
       constexpr uintptr_t kASLROffset = AslrAddress(0x1000000000ULL);
 
+      #endif
+
     #elif defined(ARCH_CPU_PPC64)
 
       #if defined(OS_AIX)
@@ -167,7 +178,8 @@ constexpr uintptr_t AslrMask(uintptr_t bits) {
 
       #endif  // !defined(OS_SOLARIS) && !defined(OS_AIX)
 
-    #endif  // !defined(ARCH_CPU_X86_64) && !defined(ARCH_CPU_PPC64) && !defined(ARCH_CPU_S390X) && !defined(ARCH_CPU_S390)
+    #endif  // !defined(ARCH_CPU_X86_64) && !defined(ARCH_CPU_PPC64) &&
+            // !defined(ARCH_CPU_S390X) && !defined(ARCH_CPU_S390)
 
   #endif  // defined(OS_POSIX)