Clean up preconditions for PartitionAlloc large-memory tests.

- Tweak IsLargeMemoryDevice() to treat devices with _almost_ 2GiB of
  available memory as "large", to allow for e.g. some memory being
  reserved for use by devices.
- ASSERT, rather than EXPECT, that SetAddressSpaceLimit() succeeds,
  so that the test will fail fast rather than trying to run, and
  proceeding to OOM the host system.

Bug: 779645
Change-Id: I12eb4cabda28b2fbdb7ef8c5cdee9fb4a60c2781
Reviewed-on: https://chromium-review.googlesource.com/744300Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512845}

base/allocator/partition_allocator/partition_alloc_unittest.cc

@@ -36,7 +36,10 @@ std::unique_ptr<T[]> WrapArrayUnique(T* ptr) {
 const size_t kTestMaxAllocation = 4096;
 
 bool IsLargeMemoryDevice() {
-  return base::SysInfo::AmountOfPhysicalMemory() >= 2LL * 1024 * 1024 * 1024;
+  // Treat any device with 2GiB or more of physical memory as a "large memory
+  // device". We check for slightly less than 2GiB so that devices with a small
+  // amount of memory not accessible to the OS still count as "large".
+  return base::SysInfo::AmountOfPhysicalMemory() >= 2040LL * 1024 * 1024;
 }
 
 bool SetAddressSpaceLimit() {
@@ -178,7 +181,7 @@ class PartitionAllocTest : public testing::Test {
       return;
     }
 
-    EXPECT_TRUE(SetAddressSpaceLimit());
+    ASSERT_TRUE(SetAddressSpaceLimit());
 
     // Work out the number of allocations for 6 GB of memory.
     const int numAllocations = (6 * 1024 * 1024) / (allocSize / 1024);