Make SimpleStringBuilder into a non-template

So that future CLs can de-inline its methods.

We do this by asking the caller to allocate the buffer instead of
having it as a data member.

Bug: webrtc:8982
Change-Id: I246b0973e54510fdd880c3b6875336c31334d008
Reviewed-on: https://webrtc-review.googlesource.com/60000
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Fredrik Solenberg <solenberg@webrtc.org>
Reviewed-by: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22355}

rtc_base/BUILD.gn

@@ -142,12 +142,15 @@ rtc_source_set("safe_minmax") {
 
 rtc_source_set("stringutils") {
   sources = [
+    "strings/string_builder.cc",
     "strings/string_builder.h",
     "stringutils.cc",
     "stringutils.h",
   ]
   deps = [
     ":checks",
+    ":safe_minmax",
+    "../api:array_view",
   ]
 }