Do not include libplatform/libplatform.h in heap.h.

The analysis on go/chrome-includes [1] shows this to be a relatively
expensive include. Remove it, as v8 classes are already forward
declared. Then do IWYU to fix the build.

[1] https://commondatastorage.googleapis.com/chromium-browser-clang/include-analysis.html

Change-Id: I6e5f765ac053ed3010220a2ae0e5ea1c6a2ee1d4
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/81550
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
diff --git a/testing/fxgc_unittest.cpp b/testing/fxgc_unittest.cpp
index 538565e..5b0a493 100644
--- a/testing/fxgc_unittest.cpp
+++ b/testing/fxgc_unittest.cpp
@@ -7,6 +7,7 @@
 #include "fxjs/gc/heap.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "testing/v8_test_environment.h"
+#include "v8/include/libplatform/libplatform.h"
 
 FXGCUnitTest::FXGCUnitTest() = default;