Revert "metrics: Initialize persistent metric allocator early."

This reverts commit 0cdc1458.

Reason for revert: Breaks ToastHWATest* on Marshmallow 64 bit Tester:
[FATAL:persistent_histogram_allocator.cc(893)] Check failed: !subtle::NoBarrier_Load(&g_histogram_allocator).

https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Marshmallow%2064%20bit%20Tester/23305

Original change's description:
> metrics: Initialize persistent metric allocator early.
> 
> The PersistentMemoryAllocator in the gpu (and other) process is created
> asynchronously. If any uma-histogram is reported before that time, then
> those Histograms are not stored in the shared memory. This has some
> implications:
>  . If the process crashes (which is common for the gpu process on
>    android), then these metrics are lost.
>  . If the process stays alive, then the metrics are reported
>    periodically, but because it is buffered, it doesn't match up
>    correctly with the other metrics that are reported in the shared
>    memory.
> 
> The allocator is created in response to a message from the browser,
> which includes the handle for the shared memory to use for the allocator.
> The browser waits until the process has been launched and connected.
> So this happens much later, compared to other messages which are sent
> from browser over mojo without waiting for the connection to have
> been established first. Therefore, change this so that the browser
> immediately shares the shared-memory segment with the gpu process. mojo
> takes care of waiting for the process-launch etc.
> 
> BUG=865179, 778749
> 
> Change-Id: Iccd3d4422de3e22ead4bc6510afc5ea207bf6b6e
> Reviewed-on: https://chromium-review.googlesource.com/1174709
> Reviewed-by: Brian White <bcwhite@chromium.org>
> Reviewed-by: Avi Drissman <avi@chromium.org>
> Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#583673}

TBR=avi@chromium.org,sadrul@chromium.org,bcwhite@chromium.org

Change-Id: I65e9ad62e6613d0eaf08e834def5a988a60c4a19
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 865179, 778749
Reviewed-on: https://chromium-review.googlesource.com/1179041Reviewed-by: Matthew Jones <mdjones@chromium.org>
Commit-Queue: Matthew Jones <mdjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583859}

content/browser/browser_child_process_host_impl.cc

@@ -178,7 +178,6 @@ BrowserChildProcessHostImpl::BrowserChildProcessHostImpl(
 
   // Create a persistent memory segment for subprocess histograms.
   CreateMetricsAllocator();
-  ShareMetricsAllocatorToProcess();
 }
 
 BrowserChildProcessHostImpl::~BrowserChildProcessHostImpl() {
@@ -390,6 +389,7 @@ void BrowserChildProcessHostImpl::OnChannelConnected(int32_t peer_pid) {
   delegate_->OnChannelConnected(peer_pid);
 
   if (IsProcessLaunched()) {
+    ShareMetricsAllocatorToProcess();
     BrowserThread::PostTask(
         BrowserThread::UI, FROM_HERE,
         base::BindOnce(&NotifyProcessLaunchedAndConnected, data_.Duplicate()));
@@ -608,6 +608,7 @@ void BrowserChildProcessHostImpl::OnProcessLaunched() {
   delegate_->OnProcessLaunched();
 
   if (is_channel_connected_) {
+    ShareMetricsAllocatorToProcess();
     BrowserThread::PostTask(
         BrowserThread::UI, FROM_HERE,
         base::BindOnce(&NotifyProcessLaunchedAndConnected, data_.Duplicate()));