sandbox/linux/bpf_dsl: eliminate implicit dependency on C++ compiler behavior

For the expression F(G(), H()), C++ does not guarantee whether G() or
H() will be evaluated first.  Apparently the system compiler on Chrome
OS makes a different decision than on Ubuntu, which causes the golden
file tests introduced by https://crrev.com/1306723002 (which were
generated on Ubuntu) to fail on Chrome OS.

Easy fix: rewrite the expression as

    g = G()
    h = H()
    F(g, h)

to avoid any compiler behavior dependency.

BUG=529480

Review URL: https://codereview.chromium.org/1327693005

Cr-Commit-Position: refs/heads/master@{#347838}

sandbox/linux/bpf_dsl/bpf_dsl.cc

@@ -118,8 +118,11 @@ class IfThenResultExprImpl : public internal::ResultExprImpl {
       : cond_(cond), then_result_(then_result), else_result_(else_result) {}
 
   CodeGen::Node Compile(PolicyCompiler* pc) const override {
-    return cond_->Compile(
-        pc, then_result_->Compile(pc), else_result_->Compile(pc));
+    // We compile the "then" and "else" expressions in separate statements so
+    // they have a defined sequencing.  See https://crbug.com/529480.
+    CodeGen::Node then_node = then_result_->Compile(pc);
+    CodeGen::Node else_node = else_result_->Compile(pc);
+    return cond_->Compile(pc, then_node, else_node);
   }
 
   bool HasUnsafeTraps() const override {