Revert "Set is_multi_dll_chrome to false by default"

This reverts commit e0dffd84.

Reason for revert: Broke Windows archive bots. (Slightly speculative)

https://ci.chromium.org/p/chromium/builders/ci/win-archive-rel/6444
https://ci.chromium.org/p/chromium/builders/ci/win32-archive-rel/6062

Note, the error comes from headless/, where the build config to handle
multi-DLL is very fragile.

Original change's description:
> Set is_multi_dll_chrome to false by default
> 
> This change sets is_multi_dll_chrome to false by default thus merging
> chrome.dll and chrome_child.dll to a single dll.  This saves disk space
> and improves startup performance.  It is also consistent with all other
> platforms (windows was the only platform the split was enabled by
> default).
> 
> Bug: 726150
> Change-Id: Iade2c7f1bbe696d45ebac7483072f7e2e7e84ab4
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1850272
> Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
> Reviewed-by: Will Harris <wfh@chromium.org>
> Reviewed-by: Nico Weber <thakis@chromium.org>
> Commit-Queue: Nico Weber <thakis@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#705667}

TBR=gab@chromium.org,thakis@chromium.org,brucedawson@chromium.org,wfh@chromium.org,chrdavis@microsoft.com

Change-Id: I8dd4c4c075c855e1ce68e08c766eab34374ed3e1
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 726150
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1860903Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705680}

build/config/chrome_build.gni

@@ -10,8 +10,7 @@ declare_args() {
 
   # Break chrome.dll into multple pieces based on process type. Only available
   # on Windows.
-  # TODO: Remove this flag once m79 has shipped with this disabled.
-  is_multi_dll_chrome = false
+  is_multi_dll_chrome = is_win && !is_component_build
 
   # Turn this on to generate order files. See
   # https://chromium.googlesource.com/chromium/src/+/master/docs/win_order_files.md