Revert "Use /O2 everywhere during both PGO phase 1 and 2"

This reverts commit 3b4a4ace.

Reason for revert: Increases the size of official chrome.dll by 3.5%

Original change's description:
> Use /O2 everywhere during both PGO phase 1 and 2
>
> This greatly reduces object file size during PGO phase 1 (20GB -> 9GB),
> and makes the linker use a lot less memory (160+GB -> 7.5GB private
> bytes).
>
> Bug: 1058040
> Change-Id: I7dd433b46afab4b4c52b63d5698c75cad206f407
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2427438
> Reviewed-by: Sébastien Marchand <sebmarchand@chromium.org>
> Reviewed-by: Nico Weber <thakis@chromium.org>
> Commit-Queue: Reid Kleckner <rnk@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#810282}

TBR=thakis@chromium.org,rnk@chromium.org,sebmarchand@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 1058040, 1132257
Change-Id: Iad463791d95e0172524f229314dfe98b04860553
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2431703Reviewed-by: Reid Kleckner <rnk@chromium.org>
Reviewed-by: Sébastien Marchand <sebmarchand@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Reid Kleckner <rnk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810882}

build/config/compiler/BUILD.gn

@@ -2000,7 +2000,7 @@ config("default_stack_frames") {
 # Default "optimization on" config.
 config("optimize") {
   if (is_win) {
-    if (chrome_pgo_phase == 0) {
+    if (chrome_pgo_phase != 2) {
       # Favor size over speed, /O1 must be before the common flags.
       # /O1 implies /Os and /GF.
       cflags = [ "/O1" ] + common_optimize_on_cflags + [ "/Oi" ]