Fix up stack limit sanity assertions.

MozReview-Commit-ID: ILblsins91P

components/style/parallel.rs

@@ -34,6 +34,22 @@ use traversal::{DomTraversal, PerLevelTraversalData};
 /// The minimum stack size for a thread in the styling pool, in kilobytes.
 pub const STYLE_THREAD_STACK_SIZE_KB: usize = 128;
 
+/// The stack margin. If we get this deep in the stack, we will skip recursive
+/// optimizations to ensure that there is sufficient room for non-recursive work.
+///
+/// When measured with 128KB stacks and 40KB margin, we could support 53
+/// levels of recursion before the limiter kicks in, on x86_64-Linux [1].
+///
+/// We currently use 45KB margins, because that seems to be the minimum amount
+/// of head room required by an unoptimized debug build, as measured on [2]. We
+/// could probably get away with a smaller margin on optimized release builds,
+/// but that would be a pain, and the extra padding reduces stability risk.
+///
+/// [1] See Gecko bug 1376883 for more discussion on the measurements.
+/// [2] layout/style/crashtests/1383981-3.html
+///
+pub const STACK_SAFETY_MARGIN_KB: usize = 45;
+
 /// The maximum number of child nodes that we will process as a single unit.
 ///
 /// Larger values will increase style sharing cache hits and general DOM