fix(chrome installers): Check for existence

- Don't launch Chrome installers if they're already
    installed
 - Heuristic for Canary is a path check
 - Heuristic for dev channel is path check + reg key

GoogleChrome.Canary/GoogleChrome.Canary.nuspec

@@ -24,6 +24,7 @@
     <releaseNotes></releaseNotes>
   </metadata>
   <files>
+    <file src="..\core\ChromeHelpers.ps1" target="tools" />
     <file src="tools\**" target="tools" />
   </files>
 </package>

GoogleChrome.Canary/tools/chocolateyInstall.ps1

@@ -2,9 +2,25 @@ $package = 'Chrome.Canary'
 
 try {
 
-  $url = 'https://dl.google.com/tag/s/appguid%3D%7B4ea16ac7-fd5a-47c3-875b-dbf4a2008c20%7D%26iid%3D%7B0281A7E2-6043-D983-8BBA-7FD622493C9D%7D%26lang%3Den%26browser%3D4%26usagestats%3D1%26appname%3DGoogle%2520Chrome%2520Canary%26needsadmin%3Dfalse/update2/installers/ChromeSetup.exe'
+  function Get-CurrentDirectory
+  {
+    $thisName = $MyInvocation.MyCommand.Name
+    [IO.Path]::GetDirectoryName((Get-Content function:$thisName).File)
+  }
 
-  Install-ChocolateyPackage 'ChromeSetup' 'exe' '' $url
+  . (Join-Path (Get-CurrentDirectory) 'ChromeHelpers.ps1')
+
+  $installedPath = Get-ChromePath -Canary
+  if (Test-Path $installedPath)
+  {
+    Write-Host "Chrome Canary already installed at $installedPath"
+  }
+  else
+  {
+    $url = 'https://dl.google.com/tag/s/appguid%3D%7B4ea16ac7-fd5a-47c3-875b-dbf4a2008c20%7D%26iid%3D%7B0281A7E2-6043-D983-8BBA-7FD622493C9D%7D%26lang%3Den%26browser%3D4%26usagestats%3D1%26appname%3DGoogle%2520Chrome%2520Canary%26needsadmin%3Dfalse/update2/installers/ChromeSetup.exe'
+
+    Install-ChocolateyPackage 'ChromeSetup' 'exe' '' $url
+  }
 
   Write-ChocolateySuccess $package
 } catch {