Switches Android to ThinLTO and optimizes for space (#179)

ThinLTO is supposed to be faster during linking.

The LTO optimization level is decreased to reduce inlining and hence binary size (Chrome uses the same LTO-O level on Android for the same reason).

This reduces libflutter.so by 122,800 Bytes (120KB) uncompressed, which now comes in at 5,558,036 Bytes.

Author: goderbauer

build/config/compiler/BUILD.gn

@@ -810,6 +810,7 @@ if (is_win) {
 # Default "optimization on" config. On Windows, this favors size over speed.
 config("optimize") {
   lto_flags = []
+  ld_lto_flags = []
   if (is_win) {
     # Favor size over speed, /O1 must be before the common flags. The GYP
     # build also specifies /Os and /GF but these are implied by /O1.
@@ -819,12 +820,13 @@ config("optimize") {
   } else if (is_android) {
     cflags = [ "-Oz" ] + common_optimize_on_cflags  # Favor size over speed.
     if (enable_lto) {
-      lto_flags += [ "-flto" ]
+      lto_flags += [ "-flto=thin" ]
+      ld_lto_flags += [ "-Wl,--lto-O0" ]  # Favor size over speed.
     }
   } else {
     cflags = [ "-O2" ] + common_optimize_on_cflags
   }
-  ldflags = common_optimize_on_ldflags + lto_flags
+  ldflags = common_optimize_on_ldflags + lto_flags + ld_lto_flags
   cflags += lto_flags
 }