Use python-is-python2 where possible in install_build_deps.sh.

Distros are increasingly moving away from having a bare
`python` package that installs a python2 binary as `/usr/bin/python`.

This CL switches the dependency to use `python-is-python2` where
possible as a result.

Bug: 1182059
Bug: 1112471
Change-Id: I1082375ea433255fa0014fbffcae1628f063c111
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2824354
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Cr-Commit-Position: refs/heads/master@{#872128}

build/install-build-deps.sh

@@ -204,8 +204,7 @@ dev_list="\
   patch
   perl
   pkg-config
-  python
-  python-dev
+  python2-dev
   python-setuptools
   rpm
   ruby
@@ -219,6 +218,12 @@ dev_list="\
   $chromeos_dev_list
 "
 
+if package_exists python-is-python2; then
+  dev_list="${dev_list} python-is-python2"
+else
+  dev_list="${dev_list} python"
+fi
+
 # 64-bit systems need a minimum set of 32-bit compat packages for the pre-built
 # NaCl binaries.
 if file -L /sbin/init | grep -q 'ELF 64-bit'; then
@@ -335,6 +340,7 @@ backwards_compatible_list="\
   libgtk2.0-dev
   mesa-common-dev
   msttcorefonts
+  python-dev
   ttf-dejavu-core
   ttf-indic-fonts
   ttf-kochi-gothic