build: use higher memory estimate for Windows ThinLTO links

We were seeing out of memory errors on Windows ThinLTO bots. After
measuring memory consumption of linking chrome.dll and
chrome_child.dll, it became clear that the estimated memory usage of a
thin link was too low. Adjusting to a more realistic value.

R=dpranke,gbiv,thakis
BUG=967074

Change-Id: I7d43d38e28e351a2dc16d79abbf332972cf66e9c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1785680Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: George Burgess <gbiv@chromium.org>
Commit-Queue: Bob Haarman <inglorion@chromium.org>
Auto-Submit: Bob Haarman <inglorion@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693844}

build/toolchain/concurrent_links.gni

@@ -21,10 +21,14 @@ declare_args() {
 
 if (concurrent_links == -1) {
   if (use_thin_lto) {
-    _args = [
-      "--mem_per_link_gb=10",
-      "--reserve_mem_gb=10",
-    ]
+    _args = [ "--reserve_mem_gb=10" ]
+    if (is_win) {
+      # Based on measurements of linking chrome.dll and chrome_child.dll, plus
+      # a little padding to account for future growth.
+      _args += [ "--mem_per_link_gb=45" ]
+    } else {
+      _args += [ "--mem_per_link_gb=10" ]
+    }
   } else if (use_sanitizer_coverage || use_fuzzing_engine) {
     # Sanitizer coverage instrumentation increases linker memory consumption
     # significantly.