[Heap] Fix ArrayBuffer_SemiSpaceCopyMultipleTasks to respect PPC page size

The testcase allocates JSArraybuffer on 2 separate pages which should be
on the New space. In the testcase semi space size is set to 2MB.
Since page size on PPC is 4MB the semi new space size defaults to 4MB.
Therefore when allocating 2nd buffer, scavenge GC kicks in as from-space is
filled up and copies 1st buffer to to-space. Now, the 2nd buffer also gets
allocated on the same to-space, therefore both buffer end up being on the
same page. This fix should allocate enough semi new space to contain 2
pages (for all platform).
The testcase was added as part of https://codereview.chromium.org/2036643002

R=mlippautz@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com, mbrandy@us.ibm.com

BUG=
LOG=N

Review-Url: https://codereview.chromium.org/2167853002
Cr-Commit-Position: refs/heads/master@{#38028}

test/cctest/heap/test-array-buffer-tracker.cc

@@ -292,7 +292,7 @@ UNINITIALIZED_TEST(ArrayBuffer_SemiSpaceCopyMultipleTasks) {
   // Test allocates JSArrayBuffer on different pages before triggering a
   // full GC that performs the semispace copy. If parallelized, this test
   // ensures proper synchronization in TSAN configurations.
-  FLAG_min_semi_space_size = 2;
+  FLAG_min_semi_space_size = 2 * Page::kPageSize / MB;
   v8::Isolate::CreateParams create_params;
   create_params.array_buffer_allocator = CcTest::array_buffer_allocator();
   v8::Isolate* isolate = v8::Isolate::New(create_params);